Display device, display module, and electronic device

ABSTRACT

A novel display device or the like in which a transistor connected to a scan line has small gate capacitance is provided. A novel display device or the like in which a scan line has low resistance is provided. A novel display device or the like in which pixels can be arranged with high density is provided. A novel display device or the like that can be manufactured without an increase in cost is provided. In a transistor including a first gate electrode and a second gate electrode, the first gate electrode is formed using a metal material with low resistance and the second gate electrode is formed using a metal oxide material that can reduce oxygen vacancies in an oxide semiconductor layer. The first gate electrode is connected to the scan line, and the second gate electrode is connected to a wiring to which a constant potential is supplied.

TECHNICAL FIELD

One embodiment of the present invention relates to a display deviceprovided with a transistor including an oxide semiconductor.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. One embodiment of thepresent invention relates to a process, a machine, manufacture, or acomposition of matter. In particular, one embodiment of the presentinvention relates to a semiconductor device, a display device, alight-emitting device, a power storage device, a storage device, adriving method thereof, or a manufacturing method thereof.

BACKGROUND ART

A technique for forming a transistor using a semiconductor layer formedover a substrate having an insulating surface (also referred to as afield-effect transistor (FET) or a thin film transistor (TFT)) has beenattracting attention. The transistor is used in a wide range ofelectronic devices such as an integrated circuit (IC) and an imagedisplay device (display device). Semiconductor materials typified bysilicon are widely known as materials for semiconductor layers that canbe used for transistors. As other materials, oxide semiconductors havebeen attracting attention.

For example, a technique for forming a transistor using an amorphousoxide containing In, Zn, Ga, Sn, and the like as an oxide semiconductoris disclosed (see Patent Document 1). A technique for forming aself-aligned top-gate transistor using an oxide layer is also disclosed(see Patent Document 2). In addition, a technique for forming atransistor in which an oxide layer where a channel is formed iselectrically surrounded by electric fields of top and bottom gateelectrodes to achieve high field-effect mobility is disclosed (seePatent Document 3).

In addition, a technique for forming a transistor with high electricalreliability, e.g., a small shift in threshold voltage by using aninsulating layer that releases oxygen by being heated as a baseinsulating layer of an oxide semiconductor layer where a channel isformed, to reduce oxygen vacancies in the oxide semiconductor layer isdisclosed (see Patent Document 4).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2006-165529

[Patent Document 2] Japanese Published Patent Application No.2009-278115

[Patent Document 3] Japanese Published Patent Application No.2014-241404

[Patent Document 4] Japanese Published Patent Application No.2012-009836

DISCLOSURE OF INVENTION

Transistors including oxide layers are expected to be used for displaydevices. Such transistors are required to have high field-effectmobility and high reliability. To achieve high field-effect mobility, itis effective in a transistor to electrically surround an oxide layerwhere a channel is formed. However, there is a problem in that the gatecapacitance of such a transistor in which an oxide layer where a channelis formed is electrically surrounded by electric fields of gateelectrodes is excessively increased when the transistor is driven with asignal of a scan line.

To decrease gate capacitance, a single-gate structure, rather than astructure in which an oxide layer is surrounded by gate electrodes, iseffectively used. However, the use of a gate electrode that is formedwith, for example, an oxide layer and releases oxygen by being heated asa top gate to achieve high reliability causes a problem of increasingthe resistance of the gate electrode, or the resistance of a scan line,compared with the use of a gate electrode formed with metal.

Such a structure in which a gate electrode that releases oxygen by beingheated is used as a top gate effectively allows a transistor to havehigh reliability. Therefore, to reduce the resistance of a scan line inthis structure, it is effective to use a gate electrode formed withmetal as a bottom gate and to form the scan line using a metal wiring onthe bottom gate side. However, such a structure has a problem in thatforming an opening for connecting a top gate and a bottom gate in asmall region such as a pixel region makes the layout difficult,resulting in difficulty in fabricating a high-definition display device.Although a metal wiring can be stacked over a gate electrode thatreleases oxygen by being heated so that the resistance of a scan line isreduced, this structure has a problem in that the number of stepsincreases and thus the manufacturing cost increases.

In view of the above problems, an object of one embodiment of thepresent invention is to provide a novel display device or the like inwhich a transistor connected to a scan line has small gate capacitance.Another object of one embodiment of the present invention is to providea novel display device or the like in which a scan line has lowresistance. Another object of one embodiment of the present invention isto provide a novel display device or the like in which pixels can bearranged with high density. Another object of one embodiment of thepresent invention is to provide a novel display device or the like thatcan be manufactured without an increase in cost.

Note that the objects of one embodiment of the present invention are notlimited to the above objects. The objects described above do not disturbthe existence of other objects. The other objects are the ones that arenot described above and will be described below. The other objects willbe apparent from and can be derived from the description of thespecification, the drawings, and the like by those skilled in the art.One embodiment of the present invention achieves at least one of theaforementioned objects and the other objects.

One embodiment of the present invention is a display device including afirst transistor, a second transistor, a first wiring, and a secondwiring. The first transistor includes a first gate electrode, a secondgate electrode, and a first semiconductor layer. The second transistorincludes a first gate electrode, a second gate electrode, and a secondsemiconductor layer. The first wiring is configured to transmit a signalfor controlling conduction states of the first transistor and the secondtransistor. The second wiring is configured to transmit a constantvoltage. The first gate electrode of the first transistor and the firstgate electrode of the second transistor are electrically connected tothe first wiring. The second gate electrode of the first transistor andthe second gate electrode of the second transistor are electricallyconnected to the second wiring. The first semiconductor layer and thesecond semiconductor layer include an oxide semiconductor. The firstgate electrode of the first transistor and the first gate electrode ofthe second transistor include a metal material. The second gateelectrode of the first transistor and the second gate electrode of thesecond transistor include a metal oxide material.

One embodiment of the present invention is a display device including afirst transistor, a second transistor, a third transistor, a firstwiring, and a second wiring. The first transistor includes a first gateelectrode, a second gate electrode, and a first semiconductor layer. Thesecond transistor includes a first gate electrode, a second gateelectrode, and a second semiconductor layer. The third transistorincludes a first gate electrode, a second gate electrode, and a thirdsemiconductor layer. The first wiring is configured to transmit a signalfor controlling conduction states of the first transistor and the secondtransistor. The second wiring is configured to transmit a constantvoltage. The first gate electrode of the first transistor and the firstgate electrode of the second transistor are electrically connected tothe first wiring. The second gate electrode of the first transistor andthe second gate electrode of the second transistor are electricallyconnected to the second wiring. The first gate electrode of the thirdtransistor and the second gate electrode of the third transistor areelectrically connected to each other. The first semiconductor layer, thesecond semiconductor layer, and the third semiconductor layer include anoxide semiconductor. The first gate electrode of the first transistor,the first gate electrode of the second transistor, and the first gateelectrode of the third transistor include a metal material. The secondgate electrode of the first transistor, the second gate electrode of thesecond transistor, and the second gate electrode of the third transistorinclude a metal oxide material.

One embodiment of the present invention is a display device including afirst transistor, a second transistor, a third transistor, a capacitor,a light-emitting element, a first wiring, and a second wiring. The firsttransistor includes a first gate electrode, a second gate electrode, anda first semiconductor layer. The second transistor includes a first gateelectrode, a second gate electrode, and a second semiconductor layer.The third transistor includes a first gate electrode, a second gateelectrode, and a third semiconductor layer. The first wiring isconfigured to transmit a signal for controlling conduction states of thefirst transistor and the second transistor. The second wiring isconfigured to transmit a constant voltage. The first gate electrode ofthe first transistor and the first gate electrode of the secondtransistor are electrically connected to the first wiring. The secondgate electrode of the first transistor and the second gate electrode ofthe second transistor are electrically connected to the second wiring.One of a source and a drain of the first transistor is electricallyconnected to the first gate electrode of the third transistor, oneelectrode of the capacitor, and the second gate electrode of the thirdtransistor. One of a source and a drain of the second transistor iselectrically connected to one of a source and a drain of the thirdtransistor, the other electrode of the capacitor, and one electrode ofthe light-emitting element. The first semiconductor layer, the secondsemiconductor layer, and the third semiconductor layer include an oxidesemiconductor. The first gate electrode of the first transistor, thefirst gate electrode of the second transistor, and the first gateelectrode of the third transistor include a metal material. The secondgate electrode of the first transistor, the second gate electrode of thesecond transistor, and the second gate electrode of the third transistorinclude a metal oxide material.

One embodiment of the present invention is a display device including apixel electrically connected to a first wiring and a second wiring. Thepixel includes a first transistor and a second transistor. The firsttransistor includes a first gate electrode, a second gate electrode, anda first semiconductor layer. The second transistor includes a first gateelectrode, a second gate electrode, and a second semiconductor layer.The first wiring is configured to transmit a signal for controllingconduction states of the first transistor and the second transistor. Thesecond wiring is configured to transmit a constant voltage. The firstgate electrode of the first transistor and the first gate electrode ofthe second transistor are electrically connected to the first wiring.The second gate electrode of the first transistor and the second gateelectrode of the second transistor are electrically connected to thesecond wiring. The first semiconductor layer and the secondsemiconductor layer include an oxide semiconductor. The first gateelectrode of the first transistor and the first gate electrode of thesecond transistor include a metal material. The second gate electrode ofthe first transistor and the second gate electrode of the secondtransistor include a metal oxide material.

In the display device of one embodiment of the present invention, theoxide semiconductor preferably includes oxygen, In, Zn, and M (M is Al,Ga, Y, or Sn).

In the display device of one embodiment of the present invention, theoxide semiconductor preferably includes a crystal part having c-axisalignment.

In the display device of one embodiment of the present invention, themetal oxide material preferably includes oxygen, In, Zn, and M (M is Al,Ga, Y, or Sn), and the metal oxide material preferably has a highercarrier density than the oxide semiconductor.

Note that other embodiments of the present invention will be describedin the following embodiments with reference to the drawings.

One embodiment of the present invention can provide a novel displaydevice or the like in which a transistor connected to a scan line hassmall gate capacitance. Another embodiment of the present invention canprovide a novel display device or the like in which a scan line has lowresistance. Another embodiment of the present invention can provide anovel display device or the like in which pixels can be arranged withhigh density. Another embodiment of the present invention can provide anovel display device or the like that can be manufactured without anincrease in cost.

Note that the effects of one embodiment of the present invention are notlimited to the above effects. The effects described above do not disturbthe existence of other effects. The other effects are the ones that arenot described above and will be described below. The other effects willbe apparent from and can be derived from the description of thespecification, the drawings, and the like by those skilled in the art.One embodiment of the present invention has at least one of theaforementioned effects and the other effects. Accordingly, oneembodiment of the present invention does not have the aforementionedeffects in some cases.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are a circuit diagram and a timing chart of a displaydevice of one embodiment.

FIGS. 2A to 2C are a top view and cross-sectional views of a displaydevice of one embodiment.

FIG. 3 is a top view of a display device of one embodiment.

FIG. 4 is a perspective view of a display device of one embodiment.

FIGS. 5A and 5B are cross-sectional views of a display device of oneembodiment.

FIGS. 6A and 6B are top views of a display device of one embodiment.

FIGS. 7A and 7B are circuit diagrams of a display device of oneembodiment.

FIGS. 8A to 8C are each a circuit diagram of a display device of oneembodiment.

FIG. 9 illustrates a structure example of a display device of oneembodiment.

FIG. 10 illustrates a structure example of a display device of oneembodiment.

FIG. 11 illustrates a structure example of a display device of oneembodiment.

FIG. 12 illustrates a structure example of a touch panel of oneembodiment.

FIGS. 13A to 13D illustrate a method for manufacturing a display deviceof one embodiment.

FIGS. 14A to 14D illustrate the method for manufacturing a displaydevice of one embodiment.

FIGS. 15A to 15D illustrate the method for manufacturing a displaydevice of one embodiment.

FIGS. 16A and 16B illustrate the method for manufacturing a displaydevice of one embodiment.

FIGS. 17A to 17C illustrate the method for manufacturing a displaydevice of one embodiment.

FIGS. 18A to 18F illustrate electronic devices of embodiments.

FIGS. 19A to 19I illustrate electronic devices of embodiments.

FIGS. 20A to 20F illustrate electronic devices of embodiments.

FIGS. 21A to 21E illustrate electronic devices of embodiments.

FIGS. 22A to 22C illustrate electronic devices of embodiments.

FIG. 23 is an energy band diagram of a transistor including an oxidesemiconductor film in a channel region.

FIGS. 24A and 24B are each a circuit diagram of a display device of oneembodiment.

FIGS. 25A to 25B are each a circuit diagram of a display device of oneembodiment.

FIGS. 26A and 26B are a block diagram and a circuit diagram of Example.

FIG. 27 is a top view of Example.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments and an example will be described in detail with reference tothe drawings. Note that the present invention is not limited to thefollowing description. It will be readily appreciated by those skilledin the art that modes and details of the present invention can bemodified in various ways without departing from the spirit and scope ofthe present invention. Thus, the present invention should not beconstrued as being limited to the description in the followingembodiments and example.

Note that in structures of the present invention described below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and the descriptionthereof is not repeated. The same hatching pattern is applied toportions having similar functions, and the portions are not especiallydenoted by reference numerals in some cases.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, the size, the layer thickness, or theregion is not necessarily limited to the illustrated scale.

Note that in this specification and the like, ordinal numbers such as“first” and “second” are used in order to avoid confusion amongcomponents and do not limit the number.

A transistor is a kind of semiconductor elements and can achieveamplification of current or voltage, switching operation for controllingconduction or non-conduction, or the like. A transistor in thisspecification includes, in its category, an insulated-gate field effecttransistor (IGFET) and a thin film transistor (TFT).

Functions of a “source” and a “drain” are sometimes replaced with eachother when a transistor of opposite polarity is used or when thedirection of current flow is changed in circuit operation, for example.Therefore, the terms “source” and “drain” can be replaced with eachother in this specification.

Embodiment 1

In this embodiment, a configuration example of a display device of oneembodiment of the present invention will be described.

Configuration Example of Circuit Diagram

FIG. 1A is a circuit diagram of a pixel included in a display device.

A pixel PIX includes a transistor M1, a transistor M2, a transistor M3,a capacitor C1, and a light-emitting element EL. The pixel PIX isconnected to a scan line GL, a signal line SL, a current supply lineANODE, a wiring V0, and a common wiring CATHODE. The pixel PIXcorresponds to a subpixel included in a pixel that performs colordisplay. Although description will be given assuming that thetransistors M1 to M3 are n-channel transistors, the transistors M1 to M3may be p-channel transistors.

The scan line GL is a wiring for supplying a scan signal to a pixel. Ascan signal is a signal for controlling the conduction state of atransistor that is supplied with the scan signal. The signal line SL isa wiring for supplying a signal corresponding to image data to thepixel. The current supply line ANODE and the common wiring CATHODE arewirings for supplying current to the light-emitting element EL. Thewiring V0 is a wiring to which a constant voltage is applied.

In the transistors M1 and M2, gate electrodes are provided over andbelow a semiconductor layer. The gate electrode located below thesemiconductor layer and formed using a metal material is referred to asa first gate electrode (also referred to as a bottom gate electrode).The gate electrode located over the semiconductor layer and formed usinga metal oxide material is referred to as a second gate electrode (alsoreferred to as a top gate electrode). Structure examples that can beused for the transistors M1 and M2 will be described later. Although thetransistor M3 has the same structure as those of the transistors M1 andM2 in FIG. 1A, the structure of the transistor M3 is not limitedthereto. Note that the metal oxide material includes a metal element andoxygen.

The first gate electrode of the transistor M1 is connected to the scanline GL. The second gate electrode of the transistor M1 is connected tothe wiring V0. One of a source and a drain of the transistor M1 isconnected to the signal line SL. The other of the source and the drainof the transistor M1 is connected to a first gate electrode and a secondgate electrode of the transistor M3 and one electrode of the capacitorC1.

The first gate electrode of transistor M2 is connected to the scan lineGL. The second gate electrode of the transistor M2 is connected to thewiring V0. One of a source and a drain of the transistor M2 is connectedto the wiring V0. The other of the source and the drain of thetransistor M2 is connected to one of a source and a drain of thetransistor M3, the other electrode of the capacitor C1, and oneelectrode of the light-emitting element EL.

The first gate electrode of the transistor M3 is connected to the otherof the source and the drain of the transistor M1, the second gateelectrode of the transistor M3, and the one electrode of the capacitorC1. The one of the source and the drain of the transistor M3 isconnected to the other of the source and the drain of the transistor M2,the other electrode of the capacitor C1, and the one electrode of thelight-emitting element EL. The other of the source and the drain of thetransistor M3 is connected to the current supply line ANODE.

The one electrode of the capacitor C1 is connected to the other of thesource and the drain of the transistor M1 and the first gate electrodeand the second gate electrode of the transistor M3. The other electrodeof the capacitor C1 is connected to the other of the source and thedrain of the transistor M2, the one of the source and the drain of thetransistor M3 and the one electrode of the light-emitting element EL.

The one electrode of the light-emitting element EL is connected to theother of the source and the drain of the transistor M2, the one of thesource and the drain of the transistor M3, and the other electrode ofthe capacitor C1. The other electrode of the light-emitting element ELis connected to the common wiring CATHODE.

The scan line GL, to which the first gate electrode of the transistor M1and the first gate electrode of the transistor M2 are connected, isformed with the metal material below the semiconductor layer. The scanline GL is connected to the first gate electrode of the transistor M1and the first gate electrode of the transistor M2 without passingthrough an opening. The wiring V0, to which the second gate electrode ofthe transistor M1 and the second gate electrode of the transistor M2 areconnected, is formed with a metal material included in a conductivelayer above the transistors M1 and M2. The wiring V0 is connected to thesecond gate electrode of the transistor M1 and the second gate electrodeof the transistor M2 through an opening.

FIG. 1B is a timing chart briefly showing the operation of the circuitin FIG. 1A. FIG. 1B shows the voltage of the wiring V0 and an imagesignal of the signal line SL in one scan line selection period(P_(SCAN)) of a scan line GL(n) in an n-th row.

As shown in FIG. 1B, the image signal of the signal line in SL inP_(SCAN) is switched from a signal DATA(n−1) of a (n−1)-th row to asignal DATA(n) of the n-th row. During this period, the voltage of thewiring V0 is a constant voltage V₀.

In the above configuration, the first gate electrode and the second gateelectrode are not connected to each other in the transistors M1 and M2.Owing to the configuration, the gate capacitance between the scan lineGL and the transistors is formed only between the scan line GL and thefirst gate electrodes in contrast to the case where the first and secondgate electrodes are connected to each other. The gate capacitancebetween the wiring V0 and the transistors M1 and M2 is negligiblebecause the constant voltage is applied to the wiring V0. Thus, theabove configuration can reduce the gate capacitance between the scanline GL and the transistors compared with the case where the first andsecond gate electrodes are connected to each other. Furthermore, bycontrolling the constant voltage V₀ that is applied to the wiring V0,the threshold voltages of the transistors M1 and M2 can be adjusted.

Furthermore, in the above configuration, the scan line GL that is formedwith the metal material can be provided in the same layer as the firstgate electrode in the transistors M1 and M2. Thus, even when the firstgate electrode is formed with a conductive layer including the metalmaterial and the second gate electrode is formed with a conductive layerincluding the metal oxide material such as an oxide semiconductor, aproblem of an increase in the resistance of the scan line GL can beavoided. Moreover, manufacturing cost can be reduced by the cost forproviding an extra wiring using a metal material to reduce theresistance of the scan line GL.

Since in the above configuration, the first gate electrode can be formedwith the conductive layer including the metal material and the secondgate electrode can be formed with the conductive layer including themetal oxide material such as an oxide semiconductor, a gate electrodethat releases oxygen by being heated is used as the second gateelectrode, so that the reliability of the transistors can be improved.In addition, since the first gate electrode and the second gateelectrode are not connected to each other in the transistors M1 and M2,which means that the gate electrodes are not connected to each other ina small region such as a pixel region, a high-definition display devicecan be fabricated.

Structure Example of Transistor

Here, a structure example of a transistor that can be used for thetransistors M1 and M2 will be described with reference to FIGS. 2A to2C.

FIGS. 2A to 2C illustrate an example of a semiconductor device includinga transistor. Note that the transistor illustrated in FIGS. 2A to 2C hasa structure in which gate electrodes are provided over and below asemiconductor layer.

FIG. 2A is a top view of a transistor 100. FIG. 2B is a cross-sectionalview taken along the dashed-dotted line X1-X2 in FIG. 2A. FIG. 2C is across-sectional view taken along the dashed-dotted line Y1-Y2 in FIG.2A. For clarity, some components such as an insulating layer 110 are notillustrated in FIG. 2A. As in FIG. 2A, some components are notillustrated in some cases in top views of transistors described below.In addition, the direction of the dashed-dotted line X1-X2 may bereferred to as the channel length (L) direction, and the direction ofthe dashed-dotted line Y1-Y2 may be referred to as the channel width (W)direction.

The transistor 100 illustrated in FIGS. 2A to 2C includes a conductivelayer 106 formed over a substrate 102, an insulating layer 104 formedover the conductive layer 106, an oxide semiconductor layer 108 over theinsulating layer 104, the insulating layer 110 over the oxidesemiconductor layer 108, an oxide semiconductor layer 112 over theinsulating layer 110, and an insulating layer 116 over the insulatinglayer 104 and the oxide semiconductor layers 108 and 112. Furthermore,the oxide semiconductor layer 108 has a channel region 108 i in contactwith the insulating layer 110, a source region 108 s in contact with theinsulating layer 116, and a drain region 108 d in contact with theinsulating layer 116.

The transistor 100 may further include a conductive layer 120 aelectrically connected to the source region 108 s through an opening 141a formed in the insulating layer 116, and a conductive layer 120 belectrically connected to the drain region 108 d through an opening 141b formed in the insulating layer 116.

The conductive layer 106 functions as a first gate electrode and isformed using a metal material. The oxide semiconductor layer 112functions as a second gate electrode and is formed using a metal oxidematerial. The insulating layer 104 functions as a first gate insulatinglayer, and the insulating layer 110 functions as a second gateinsulating layer.

The insulating layer 116 includes one or both of nitrogen and hydrogen.From the insulating layer 116 including nitrogen and/or hydrogen,nitrogen and/or hydrogen can be supplied to the oxide semiconductorlayer 108 and the oxide semiconductor layer 112.

As the insulating layer 116, for example, a nitride insulating layer canbe used. The nitride insulating layer can be formed using siliconnitride, silicon nitride oxide, aluminum nitride, aluminum nitrideoxide, or the like. The hydrogen concentration in the insulating layer116 is preferably higher than or equal to 1×10²² atoms/cm³.

The oxide semiconductor layer 112 has a function of supplying oxygen tothe insulating layer 110. The oxide semiconductor layer 112 having afunction of supplying oxygen to the insulating layer 110 enables theinsulating layer 110 to contain excess oxygen. When the insulating layer110 includes an excess oxygen region, excess oxygen can be supplied tothe oxide semiconductor layer 108, specifically, the channel region 108i. Therefore, a semiconductor device with high reliability can beprovided.

The insulating layer 110 can be formed to have a single-layer structureor layered structure using an oxide insulating layer or a nitrideinsulating layer. For example, the insulating layer 110 can be formed tohave a single-layer structure or layered structure using silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, aluminumoxide, hafnium oxide, gallium oxide, a Ga—Zn oxide, or the like.

Since the insulating layer 110 formed over the oxide semiconductor layer108 contains excess oxygen, excess oxygen can be selectively supplied tothe channel region 108 i. Alternatively, after excess oxygen is suppliedto the channel region 108 i, the source region 108 s, and the drainregion 108 d, the carrier density in the source region 108 s and thedrain region 108 d may be selectively increased.

Note that the thickness of the insulating layer 110 is preferablysmaller than that of the insulating layer 104. As described above, theconstant voltage is applied from the wiring V0 to the oxidesemiconductor layer 112 functioning as a second gate electrode. Thesmall thickness of the insulating layer 110 allows large parasiticcapacitance to be formed in the transistor 100 by the second gateelectrode, the second gate insulating layer, and the oxide semiconductorlayer 108. This can suppress the dielectric breakdown of the transistordue to static discharge or the like.

The carrier density in the oxide semiconductor layer 112 having suppliedoxygen to the insulating layer 110 is increased by nitrogen and/orhydrogen supplied from the insulating layer 116. In other words, theoxide semiconductor layer 112 also functions as an oxide conductor (OC).Thus, the oxide semiconductor layer 112 has a higher carrier densitythan the oxide semiconductor layer 108.

Furthermore, the oxide semiconductor layer 112 and the source region 108s and the drain region 108 d of the oxide semiconductor layer 108 mayeach include an element that forms an oxygen vacancy. Typical examplesof the element that forms an oxygen vacancy are hydrogen, boron, carbon,nitrogen, fluorine, phosphorus, sulfur, chlorine, and a rare gaselement. Typical examples of the rare gas element are helium, neon,argon, krypton, and xenon.

An impurity element added to the oxide semiconductor layer cuts a bondbetween a metal element and oxygen in the oxide semiconductor layer, sothat an oxygen vacancy is formed. Alternatively, when an impurityelement is added to the oxide semiconductor layer, oxygen bonded to ametal element in the oxide semiconductor layer is bonded to the impurityelement and released from the metal element, so that an oxygen vacancyis formed. As a result, the oxide semiconductor layer has a highercarrier density, and thus, the conductivity thereof becomes higher.

The transistor 100 preferably includes a region in which a side endportion of the insulating layer 110 is aligned with a side end portionof the oxide semiconductor layer 112. In other words, in the transistor100, an upper end portion of the insulating layer 110 is substantiallyaligned with a lower end portion of the oxide semiconductor layer 112.The above structure can be obtained by processing the insulating layer110 with the use of the oxide semiconductor layer 112 as a mask, forexample.

The oxide semiconductor layer 108 and the oxide semiconductor layer 112are each formed using a metal oxide such as In-M-Zn oxide (M is Al, Ga,Y, or Sn). Alternatively, In—Ga oxide or In—Zn oxide may each be usedfor the oxide semiconductor layer 108 and the oxide semiconductor layer112. It is particularly preferred that the oxide semiconductor layer 108and the oxide semiconductor layer 112 be formed using metal oxidescontaining the same elements because the manufacturing cost can bereduced.

In the case where the oxide semiconductor layers 108 and 112 are each anIn—M—Zn oxide, it is preferred that the atomic ratio of metal elementsof a sputtering target used to form a layer of the In-M-Zn oxide satisfyIn≥M and Zn≥M. As the atomic ratio of metal elements in such asputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:1.5,In:M:Zn=2:1:2.3, In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1,In:M:Zn=5:1:7, or the like is preferable. Note that the atomic ratios ofmetal elements in the formed oxide semiconductor layers 108 and 112 mayvary from the above atomic ratio of metal elements of the sputteringtarget within a range of approximately ±40%. For example, when asputtering target with the atomic ratio of In:Ga:Zn=4:2:4.1 is used, anatomic ratio of In:Ga:Zn in the oxide semiconductor layer may be 4:2:3and its vicinity.

When an oxide semiconductor layer with a low impurity concentration anda low density of defect states is used in the channel region 108 i, thetransistor can have more excellent electrical characteristics. Here, thestate in which the impurity concentration is low and the density ofdefect states is low (the amount of oxygen vacancies is small) isreferred to as “highly purified intrinsic” or “substantially highlypurified intrinsic”. It is also possible to call this state “intrinsic”or “substantially intrinsic”. A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor has fewcarrier generation sources, and thus has a low carrier density in somecases. Thus, a transistor including the oxide semiconductor layer inwhich a channel region is formed is likely to have a positive thresholdvoltage (normally-off characteristics). A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor layer has alow density of defect states and accordingly has a low density of trapstates in some cases. Furthermore, a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor layer has anextremely low off-state current. Thus, a transistor whose channel regionis formed in the oxide semiconductor layer has a small variation inelectrical characteristics and high reliability in some cases.

Meanwhile, the source region 108 s, the drain region 108 d, and theoxide semiconductor layer 112 are in contact with the insulating layer116. Hydrogen and/or nitrogen are/is added from the insulating layer 116to the source region 108 s, the drain region 108 d, and the oxidesemiconductor layer 112 in contact with the insulating layer 116, sothat the carrier densities in the source region 108 s, the drain region108 d, and the oxide semiconductor layer 112 are increased.

The carrier density of an oxide semiconductor layer will be describedbelow.

Examples of a factor that affects the carrier density of an oxidesemiconductor layer include oxygen vacancies (Vo) and impurities in theoxide semiconductor layer.

As the amount of oxygen vacancies in the oxide semiconductor layerincreases, the density of defect states increases when hydrogen isbonded to the oxygen vacancies (this state is also referred to as VoH).The density of defect states also increases with an increase in theamount of impurities in the oxide semiconductor layer. Hence, thecarrier density of the oxide semiconductor layer can be adjusted bycontrolling the density of defect states in the oxide semiconductorlayer.

A transistor using the oxide semiconductor layer in a channel regionwill be described.

The carrier density of the oxide semiconductor layer is preferablyreduced in order to suppress the negative shift of the threshold voltageof the transistor or reduce the off-state current of the transistor. Inorder to reduce the carrier density of the oxide semiconductor layer,the impurity concentration in the oxide semiconductor layer is reducedso that the density of defect states can be reduced. In thisspecification and the like, the state in which the impurityconcentration is low and the density of defect states is low is referredto as “highly purified intrinsic” or “substantially highly purifiedintrinsic”. The carrier density of a highly purified intrinsic oxidesemiconductor layer is lower than 8×10¹⁵ cm⁻³, preferably lower than1×10¹¹ cm⁻³, more preferably lower than 1×10¹⁰ cm⁻³ and is higher thanor equal to 1×10⁻⁹ cm⁻³.

In contrast, the carrier density of the oxide semiconductor layer ispreferably increased in order to increase the on-state current of thetransistor or improve the field-effect mobility of the transistor. Inorder to increase the carrier density of the oxide semiconductor layer,it is preferred that the impurity concentration or the density of defectstates in the oxide semiconductor layer be slightly increased, or thebandgap of the oxide semiconductor layer be narrowed. For example, anoxide semiconductor layer that has a slightly high impurityconcentration or a slightly high density of defect states in the rangewhere a favorable on/off ratio is obtained in the Id-Vg characteristicsof a transistor can be regarded as substantially intrinsic. Furthermore,an oxide semiconductor layer that has a high electron affinity and anarrow bandgap and thus has an increased density of thermally excitedelectrons (carriers) can be regarded as substantially intrinsic. Notethat a transistor using an oxide semiconductor layer with higherelectron affinity has a lower threshold voltage.

The oxide semiconductor layer with an increased carrier density hassomewhat n-type conductivity; thus, it can be referred to as a“slightly-n” oxide semiconductor layer.

The carrier density of a substantially intrinsic oxide semiconductorlayer is preferably higher than or equal to 1×10⁵ cm⁻³ and lower than1×10¹⁸ cm⁻³, more preferably higher than or equal to 1×10⁷ cm⁻³ andlower than or equal to 1×10¹⁷ cm⁻³, still more preferably higher than orequal to 1×10⁹ cm⁻³ and lower than or equal to 5×10¹⁶ cm⁻³, yet morepreferably higher than or equal to 1×10¹⁰ cm⁻³ and lower than or equalto 1×10¹⁶ cm⁻³, and yet still more preferably higher than or equal to1×10¹¹ cm⁻³ and lower than or equal to 1×10¹⁵ cm⁻³.

The use of the substantially intrinsic oxide semiconductor layerdescribed above may improve the reliability of the transistor. Here, thereason why the transistor including the oxide semiconductor layer in achannel region has high reliability will be described with reference toFIG. 23 . FIG. 23 is an energy band diagram of the transistor includingthe oxide semiconductor layer in the channel region.

In FIG. 23 , GE stands for a gate electrode, GI stands for a gateinsulating film, OS stands for an oxide semiconductor layer, and SDstands for a source electrode or a drain electrode. That is to say, FIG.23 is an example of the energy band of the gate electrode, the gateinsulating film, the oxide semiconductor layer, and the source electrodeor the drain electrode in contact with the oxide semiconductor layer.

In FIG. 23 , a silicon oxide film is used as the gate insulating film,and an In—Ga—Zn oxide is used for the oxide semiconductor layer. Inaddition, it is assumed that the transition level εf of a defect thatcan be formed in the silicon oxide film is located approximately 3.1 eVapart from the conduction band minimum of the gate insulating film, andthe Fermi level Ef of the silicon oxide film at the interface betweenthe oxide semiconductor layer and the silicon oxide film when the gatevoltage Vg is 30 V is located approximately 3.6 eV apart from theconduction band minimum of the gate insulating film. Note that the Fermilevel Ef of the silicon oxide film varies depending on the gate voltage.For example, as the gate voltage is increased, the Fermi level Ef of thesilicon oxide film at the interface between the oxide semiconductorlayer and the silicon oxide film becomes low. In FIG. 23 , hollowcircles indicate electrons (carriers), and symbols “X” indicate defectstates in the silicon oxide film.

As illustrated in FIG. 23 , for example, when the carriers are thermallyexcited under application of the gate voltage, the carriers are trappedin the defect states (“X” in the diagram), and the charge state of thedefect states is changed from positive (“+”) to neutral (“0”).Specifically, in the case where the value obtained by adding the thermalexcitation energy to the Fermi level Ef of the silicon oxide filmbecomes greater than the transition level εf of the defect, the chargestate of the defect states in the silicon oxide film is changed frompositive to neutral, and the threshold voltage of the transistor ispositively shifted.

When an oxide semiconductor layer with a different electron affinity isused, the Fermi level of the interface between the gate insulating filmand the oxide semiconductor layer might be changed. When an oxidesemiconductor layer with a greater electron affinity is used, theconduction band minimum of the gate insulating film is relatively highat the interface between the gate insulating film and the oxidesemiconductor layer or in the vicinity of the interface. In that case,the defect states (“X” in FIG. 23 ) that can be formed in the gateinsulating film are also located in a relatively high position; thus,the energy difference between the Fermi level of the gate insulatingfilm and the Fermi level of the oxide semiconductor film is increased.This results in less charge trapped in the gate insulating film. Forexample, a change in the charge states of the defect states that can beformed in the silicon oxide film is smaller; thus, a change in thethreshold voltage of the transistor due to gate bias temperature (GBT)stress can be smaller.

The above is the description of the carrier density of the oxidesemiconductor layer.

As illustrated in FIG. 2C, the oxide semiconductor layer 108 i faces theconductive layer 106 functioning as a first gate electrode and the oxidesemiconductor layer 112 functioning as a second gate electrode. That is,the oxide semiconductor layer 108 i is positioned between the conductivelayer and the oxide semiconductor layer which function as gateelectrodes.

Such a structure enables the oxide semiconductor layer 108 included inthe transistor 100 to be electrically surrounded by an electric fielddue to a scan signal of the conductive layer 106 functioning as a firstgate electrode and an electric field due to the constant voltage of theoxide semiconductor layer 112 functioning as a second gate electrode.

In the transistor 100, a scan signal for controlling the conductionstate of the transistor 100 is supplied from the first gate electrodeand the constant voltage is applied from the second gate electrode asillustrated in FIG. 1A. Since the constant voltage is applied to thewiring V0, the gate capacitance between the wiring V0 and thetransistors M1 and M2 is negligible, and the gate capacitance betweenthe scan line GL and the transistors can be reduced compared with thecase where the first and second gate electrodes are connected to eachother.

Furthermore, the conduction state of the transistor 100 is controlled bythe scan line provided in the same layer as the first gate electrode.The first gate electrode is formed using the metal material. The metalmaterial has a lower resistance than the metal oxide material of theoxide semiconductor layer 112 functioning as a second gate electrode, orthe like. Thus, the resistance of the scan line formed using the samematerial as that of the conductive layer 106 can be low.

Furthermore, the transistor 100 includes the oxide semiconductor layer112 functioning as a second gate electrode as a gate electrode thatreleases oxygen by being heated like an oxide layer. Thus, thetransistor 100 has high reliability. Since the resistance of theconductive layer 106 functioning as a first gate electrode and the scanline located in the same layer as the conductive layer 106 can bereduced as described above, a disadvantage, high resistance of thesecond gate electrode, can be offset. Moreover, the number of steps canbe reduced compared with the structure in which an oxide semiconductorlayer and a metal wiring are stacked to reduce the resistance of theoxide semiconductor layer functioning as a second gate electrode;consequently, the manufacturing cost can be reduced.

Furthermore, the transistor 100 does not have an opening for connectingthe first gate electrode and the second gate electrode. This can avoidthe necessity for providing an opening in a small region like a pixelregion. Thus, the transistor 100 is suitable for a high-definitiondisplay device.

Example of Topside Structure

Next, FIG. 3 is an example of a top view that can be used for thecircuit configuration in FIG. 1A except the structure of alight-emitting element and the like. FIG. 4 illustrates the state wherea conductive layer, a semiconductor layer, and the like arranged up anddown in the top view of FIG. 3 are separately shown and connectedthrough openings. FIG. 5A is a cross-sectional view along the dottedline P1-P2 in FIG. 3 , and FIG. 5B is a cross-sectional view along thedotted line Q1-Q2 in FIG. 3 . FIGS. 6A and 6B are top views in which thetop views of FIG. 3 including the structure of the light-emittingelement and the like are arranged.

In the top view of FIG. 3 , the scan line GL, the signal line SL, thewiring V0, the current supply line ANODE, the transistor M1, thetransistor M2, a transistor M3, and the capacitor C1 are illustrated.Insulating layers and the like in the layered structure of theconductive layer and the oxide semiconductor layer are not illustrated.

The layered structure of the oxide semiconductor layer and theconductive layer forming the wirings and the like in FIG. 3 can beunderstood from FIG. 4 and FIGS. 5A and 5B. A conductive layer 151 and aconductive layer 152 functioning as first gate electrodes are providedover a substrate SUB. Then, an oxide semiconductor layer 161, an oxidesemiconductor layer 162, and an oxide semiconductor layer 163 areprovided thereover with an insulating layer 153 functioning as a firstgate insulating layer therebetween. In addition, an oxide semiconductorlayer 171, an oxide semiconductor layer 172, and an oxide semiconductorlayer 173 functioning as second gate electrodes are provided thereoverwith an insulating layer 164 functioning as a second gate insulatinglayer therebetween. Then, a conductive layer 181, a conductive layer182, a conductive layer 183, a conductive layer 184, and a conductivelayer 185 functioning as source and drain electrodes of transistors andwirings are provided thereover with an insulating layer 174therebetween. The insulating layer 174 selectively increases the carrierdensities in the oxide semiconductor layer 161, the oxide semiconductorlayer 162, and the oxide semiconductor layer 163 and the carrierdensities in the oxide semiconductor layer 171, the oxide semiconductorlayer 172, and the oxide semiconductor layer 173 to increase theconductivity of the oxide semiconductor layers. In addition, aconductive layer 191 and a conductive layer 192 are provided over theconductive layers 181, 182, 183, 184, and 185 with an insulating layer186 and an insulating layer 187 functioning as interlayer insulatinglayers therebetween. An insulating layer 193 functioning as aninterlayer insulating layer is provided over the conductive layers 191and 192. In addition, an opening 190 is formed in the insulating layers186, 187, and 193 so as to reach the conductive layer 183. The opening190 is for connection between a pixel electrode formed later and alight-emitting element provided over the pixel electrode.

In FIGS. 3 and 4 , squares with crosses indicate openings formed in theinsulating layers. The conductive layers and the oxide semiconductorlayers in separate layers are connected through the openings as shown byarrows in FIG. 4 . In FIG. 4 , the conductive layer 151 functioning asthe scan line GL, the conductive layer 191 functioning as the signalline SL, the conductive layer 181 functioning as the wiring V0, and theconductive layer 192 functioning as the current supply line ANODE areillustrated.

As seen from FIG. 3 , FIG. 4 , and FIGS. 5A and 5B, the first gateelectrode and the second gate electrode are not connected to each otherin the transistors M1 and M2. Owing to the configuration, the gatecapacitance between the scan line GL and the transistors M1 and M2 isformed only between the scan line GL and the first gate electrodes incontrast to the case where the first and second gate electrodes areconnected to each other. Thus, the above configuration can reduce thegate capacitance between the scan line GL and the transistors comparedwith the case where the first and second gate electrodes are connectedto each other.

Furthermore, as seen from FIG. 3 , FIG. 4 , and FIGS. 5A and 5B, thescan line GL that is formed with the metal material can be provided inthe same layer as the first gate electrode in the transistors M1 and M2.Thus, even when the first gate electrode is formed with a conductivelayer including the metal material and the second gate electrode isformed with a conductive layer including the metal oxide material suchas an oxide semiconductor, a problem of an increase in the resistance ofthe scan line GL can be avoided. Moreover, manufacturing cost can bereduced by the cost for providing an extra wiring using a metal materialto reduce the resistance of the scan line GL.

In addition, as seen from FIG. 3 , FIG. 4 , and FIGS. 5A and 5B, twoelectrodes of the capacitor C1 can be formed using the conductive layer152 and the oxide semiconductor layer 163. Reducing the thickness of theinsulating layer 153 between the two electrodes can increase thecapacitance of the capacitor.

FIG. 6A is a top view of 2×3 subpixels for three colors (e.g., red (R),green (G), and blue (B)), each of which corresponds to the pixelillustrated in FIG. 3 , FIG. 4 , and FIGS. 5A and 5B. FIG. 6Aillustrates subpixels (R1, R2, G1, G2, B1, and B2) arranged in two rows,an m-th row and an (m+1)-th row, and three columns, an n-th column, an(n+1)-th column, and an (n+2)-th column. FIG. 6A also illustrates alight-emitting layer 198 included in a light-emitting element EL and apartition layer 199 as well as the opening 190 described with referenceto FIG. 3 to FIGS. 5A and 5B. FIG. 6A also illustrates a scan line GL_min the m-th row, a scan line GL_m+1 in the (m+1)-th row, a signal lineSL_n in the n-th column, a signal line SL_n+1 in the (n+1)-th column, asignal line SL_n+2 in the (n+2)-th column, the wiring V0, and thecurrent supply line ANODE.

FIG. 6B is a schematic diagram of the top view in FIG. 6A. In FIG. 6B,the light-emitting layer 198, the partition layer 199, and the like areprovided in a region 22, and a circuit including the transistors M1 andM3 and the like are provided in a region 24. As illustrated also in FIG.6A, the opening 190 is located in the vicinity of the center of theregion 24. The region 24 is located so as not to be aligned with theregion 22, whereby the opening 190 can be located at an end portion ofthe region 22. This structure allows a light-emitting region to beprovided regardless of the position of the opening 190.

Modification Example

A circuit configuration for which one embodiment of the presentinvention can be used is not limited to the pixel configurationincluding the transistors M1 to M3 in FIG. 1A. For example, oneembodiment of the present invention can also be used for a pixelconfiguration including two or less transistors as in FIG. 7A.

The pixel configuration illustrated in FIG. 7A includes a transistor M4,a transistor M5, a capacitor C2, and the light-emitting element EL. Thatis, the pixel configuration corresponds to the circuit configuration inFIG. 1A without the transistor M2.

Also in the configuration illustrated in FIG. 7A, a first gate electrodeand a second gate electrode are not connected to each other in thetransistor M4. Owing to the configuration, the gate capacitance betweenthe scan line GL and the transistors is formed only between the scanline GL and the first gate electrodes in contrast to the case where thefirst and second gate electrodes are connected to each other. The gatecapacitance between the wiring V0 and the transistor M4 is negligiblebecause the constant voltage is applied to the wiring V0. Thus, theabove configuration can reduce the gate capacitance between the scanline GL and the transistors compared with the case where the first andsecond gate electrodes are connected to each other. Furthermore, bycontrolling the constant voltage that is applied to the wiring V0, thethreshold voltage of the transistor M4 can be adjusted.

Furthermore, in the above configuration, the scan line GL that is formedwith the metal material can be provided in the same layer as the firstgate electrode in the transistor M4. Thus, even when the first gateelectrode is formed with a conductive layer including the metal materialand the second gate electrode is formed with a conductive layerincluding the metal oxide material such as an oxide semiconductor, aproblem of an increase in the resistance of the scan line GL can beavoided. Moreover, manufacturing cost can be reduced by the cost forproviding an extra wiring using a metal material to reduce theresistance of the scan line GL.

Since in the above configuration, the first gate electrode can be formedwith the conductive layer including the metal material and the secondgate electrode can be formed with the conductive layer including themetal oxide material such as an oxide semiconductor, a gate electrodethat releases oxygen by being heated is used as the second gateelectrode, so that the reliability of the transistors can be improved.In addition, since the first gate electrode and the second gateelectrode are not connected to each other in the transistor M4, whichmeans that the gate electrodes are not connected to each other in asmall region such as a pixel region, a high-definition display devicecan be fabricated.

A circuit configuration for which one embodiment of the presentinvention can be used is not limited to the pixel configurations in FIG.1A and FIG. 7A. For example, one embodiment of the present invention canalso be used for a pixel configuration including three or moretransistors as in FIG. 7B.

The pixel configuration illustrated in FIG. 7B includes a transistor M6,a transistor M7, a transistor M8, a transistor M9, a transistor M10, atransistor M11, a capacitor C3, a capacitor C4, a capacitor C5, and thelight-emitting element EL. The pixel with this configuration is operatedby the signal line SL, the current supply line ANODE, the wiring V0, thecommon wiring CATHODE, scan lines GL1 to GL4, and wirings V1 and V2. Thewirings V1 and V2 are each a wiring to which a constant voltage isapplied.

Also in the configuration illustrated in FIG. 7B, a first gate electrodeand a second gate electrode are not connected to each other in thetransistors M6 to M10. Owing to the configuration, the gate capacitancebetween the scan lines GL1 to GL4 and the transistors is formed onlybetween the scan lines GL1 to GL4 and the first gate electrodes incontrast to the case where the first and second gate electrodes areconnected to each other. The gate capacitance between the wiring V0 andthe transistors M6 to M10 is negligible because the constant voltage isapplied to the wiring V0. Thus, the above configuration can reduce thegate capacitance between the scan lines GL1 to GL4 and the transistorscompared with the case where the first and second gate electrodes areconnected to each other. Furthermore, by controlling the constantvoltage that is applied to the wiring V0, the threshold voltage of thetransistors M6 to M10 can be adjusted.

Furthermore, in the above configuration, the scan lines GL1 to GL4 thatare formed with the metal material can be provided in the same layer asthe first gate electrode in the transistors M6 to M10. Thus, even whenthe first gate electrode is formed with a conductive layer including themetal material and the second gate electrode is formed with a conductivelayer including the metal oxide material such as an oxide semiconductor,a problem of an increase in the resistance of the scan lines GL1 to GL4can be avoided. Moreover, manufacturing cost can be reduced by the costfor providing an extra wiring using a metal material to reduce theresistance of the scan lines GL1 to GL4.

Since in the above configuration, the first gate electrode can be formedwith the conductive layer including the metal material and the secondgate electrode can be formed with the conductive layer including themetal oxide material such as an oxide semiconductor, a gate electrodethat releases oxygen by being heated is used as the second gateelectrode, so that the reliability of the transistors can be improved.In addition, since the first gate electrode and the second gateelectrode are not connected to each other in the transistors M6 to M10,which means that the gate electrodes are not connected to each other ina small region such as a pixel region, a high-definition display devicecan be fabricated.

Although the first gate electrode and the second gate electrode of thetransistor M3 are not connected to each other in FIG. 1A, one embodimentof the present invention is not limited to this configuration. Forexample, the second gate electrode of the transistor M3 may be connectedto the wiring V0 as illustrated in FIG. 8A.

Alternatively, for example, the first gate electrode of the transistorM3 may be omitted as illustrated in FIG. 8B. Alternatively, for example,the first gate electrode of the transistor M3 may be connected to one ofa source and a drain of the transistor M3 as illustrated in FIG. 8C.

Also in the configurations illustrated in FIGS. 8A to 8C, the first gateelectrode and the second gate electrode are not connected to each otherin the transistors M1 and M2. Owing to the configuration, the gatecapacitance between the scan line GL and the transistors is formed onlybetween the scan line GL and the first gate electrodes in contrast tothe case where the first and second gate electrodes are connected toeach other. The gate capacitance between the wiring V0 and thetransistors M1 and M2 is negligible because the constant voltage isapplied to the wiring V0. Thus, the above configuration can reduce thegate capacitance between the scan line GL and the transistors comparedwith the case where the first and second gate electrodes are connectedto each other. Furthermore, by controlling the constant voltage that isapplied to the wiring V0, the threshold voltages of the transistors M1and M2 can be adjusted.

Furthermore, in the above configuration, the scan line GL that is formedwith the metal material can be provided in the same layer as the firstgate electrode in the transistors M1 and M2. Thus, even when the firstgate electrode is formed with a conductive layer including the metalmaterial and the second gate electrode is formed with a conductive layerincluding the metal oxide material such as an oxide semiconductor, aproblem of an increase in the resistance of the scan line GL can beavoided. Moreover, manufacturing cost can be reduced by the cost forproviding an extra wiring using a metal material to reduce theresistance of the scan line GL.

Since in the above configuration, the first gate electrode can be formedwith the conductive layer including the metal material and the secondgate electrode can be formed with the conductive layer including themetal oxide material such as an oxide semiconductor, a gate electrodethat releases oxygen by being heated is used as the second gateelectrode, so that the reliability of the transistors can be improved.In addition, since the first gate electrode and the second gateelectrode are not connected to each other in the transistors M1 and M2,which means that the gate electrodes are not connected to each other ina small region such as a pixel region, a high-definition display devicecan be fabricated.

Although both the second gate electrodes of the transistors M1 and M2are connected to the wiring V0 in FIG. 1A, one embodiment of the presentinvention is not limited to this configuration. For example, asillustrated in FIG. 24A, the second gate electrode of the transistor M1may be connected to the wiring V0, and the second gate electrode of thetransistor M2 may be connected to the scan line GL. This configurationcan increase the current supply capability of the transistor M2.

Alternatively, as illustrated in FIG. 24B, the second gate electrode ofthe transistor M2 may be connected to the wiring V0, and the second gateelectrode of the transistor M1 may be connected to the scan line GL.This configuration can increase the current supply capability of thetransistor M1.

Furthermore, instead of the scan line GL in FIG. 1A, the plurality ofscan lines GL1 and GL2 may be provided. For example, as illustrated inFIG. 25A, the first gate electrode of the transistor M1 may be connectedto the scan line GL1, and the first gate electrode of the transistor M2may be connected to the scan line GL2.

Furthermore, instead of the wiring V0 in FIG. 1A, a plurality of wiringsV0_1 and V0_2 may be provided. For example, as illustrated in FIG. 25B,the second gate electrode of the transistor M1 may be connected to thewiring V0_1, and the second gate electrode of the transistor M2 may beconnected to the wiring V0_1.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 2

In this embodiment, an example of a cross-sectional structure of adisplay device of one embodiment of the present invention will bedescribed.

Structure Example of Display Device

FIG. 9 is a schematic top view of a display device 10 described below.The display device 10 includes a pixel portion 11, a scan line drivercircuit 12, a signal line driver circuit 13, a terminal portion 15, aplurality of wirings 16 a, a plurality of wirings 16 b, and the like.

Cross-Sectional Structure Example 1-1

FIG. 10 is a schematic cross-sectional view of the display device 10.FIG. 10 shows a cross section taken along the section line A1-A2 of FIG.9 .

The display device 10 includes a first substrate 201 and a secondsubstrate 202 which are bonded to each other with a bonding layer 220.

The terminal portion 15, the wiring 16 b; a transistor 255 that isincluded in the signal line driver circuit 13, transistors 251 and 252,a capacitor 253, and a light-emitting element 254 that are included inthe pixel portion 11, and the like are provided over the first substrate201. In addition, insulating layers 211, 212, 213, and 214, a spacer215, and the like are provided over the first substrate 201.

On the first substrate 201 side of the second substrate 202, aninsulating layer 221, a light-blocking layer 231, a coloring layer 232,structures 230 a and 230 b, and the like are provided.

The light-emitting element 254 is provided over the insulating layer213. The light-emitting element 254 includes a pixel electrode 225serving as a first electrode, an EL layer 222, and a second electrode223. An optical adjustment layer 224 is provided between the pixelelectrode 225 and the EL layer 222. The insulating layer 214 covers endportions of the pixel electrode 225 and the optical adjustment layer224.

The transistor 251 functions as the transistor M1 or M2 described inEmbodiment 1 with reference to FIG. 1A. The transistor 252 functions asthe transistor M3 described in Embodiment 1 with reference to FIG. 1A.

The transistor 251, the transistor 252, and a transistor 255 areprovided with a conductive layer 275 functioning as a first gateelectrode and a conductive layer 272 functioning as a second gateelectrode. Specifically, a semiconductor in which a channel is formed issandwiched between the two gate electrodes. The conductive layer 275corresponds to the conductive layer 106 functioning as a first gateelectrode described in Embodiment 1 with reference to FIGS. 2A to 2C.The conductive layer 272 corresponds to the oxide semiconductor layer112 functioning as a second gate electrode described in Embodiment 1with reference to FIGS. 2A to 2C.

When the conductive layer 275 is used as an electrode that releasesoxygen so that oxygen vacancies in the semiconductor layer 271 can befilled, the electrical characteristics of the transistors can be stable.

In the transistor connected to a light-emitting element, like thetransistor 252, two gate electrodes are preferably electricallyconnected to each other so as to be supplied with the same signal. Sucha transistor can have higher field-effect mobility and thus have higheron-state current than other transistors. Consequently, a circuit capableof high-speed operation can be obtained.

Although the capacitor 253 is formed of part of a conductive layer 274,part of an insulating layer 217, and part of a conductive layer 273 asillustrated in FIG. 10 , the capacitor 253 can be formed of part of theconductive layer 275, part of the insulating layer 211, and part of thesemiconductor layer 271.

The light-emitting element 254 in the example illustrated in FIG. 10 isa top-emission light-emitting element. Light emission from thelight-emitting element 254 is extracted from the second substrate 202side. Such a structure enables the transistors, the capacitors, thecircuits, the wirings, and the like to be provided below thelight-emitting element 254 (i.e., on the first substrate 201 side),leading to an increase in the aperture ratio of the pixel portion 11.

The coloring layer 232 overlapping with the light-emitting element 254is provided on the surface of the second substrate 202 on the firstsubstrate 201 side. The light-blocking layer 231 may be provided in aregion where the coloring layer 232 is not provided. The light-blockinglayer 231 may overlap with the signal line driver circuit 13 asillustrated in FIG. 10 . In addition, a light-transmitting overcoatlayer may be provided to cover the coloring layer 232 and thelight-blocking layer 231.

On the first substrate 201 side of the second substrate 202, thestructure 230 a is located inward from the bonding layer 220, and thestructure 230 b is located outward from the bonding layer 220. Thestructures 230 a and 230 b each have a function of suppressingdevelopment of a crack in the insulating layer 221, the second substrate202, or the like at the end portions of the second substrate 202. Thestructures 230 a and 230 b in the example of FIG. 10 have layeredstructures including a layer formed of the same film as thelight-blocking layer 231 and a layer formed of the same film as thecoloring layer 232. Such a layered structure including two or morelayers can increase the effect of suppressing crack development.Although the structures 230 a and 230 b are provided on both sides ofthe bonding layer 220, either one of the structures 230 a and 230 b maybe provided on one side of the bonding layer 220. When there is nopossibility of cracks (e.g., when the second substrate 202 possesseshigh stiffness), the structures 230 a and 230 b may be omitted.

The spacer 215 is provided over the insulating layer 214. The spacer 215serves as a gap spacer for preventing the distance between the firstsubstrate 201 and the second substrate 202 from excessively decreasing.The angle between part of the side surface of the spacer 215 and thesurface where the spacer 215 is formed is preferably more than or equalto 45° and less than or equal to 120°, more preferably more than orequal to 60° and less than or equal to 100°, still more preferably morethan or equal to 75° and less than or equal to 90°. Owing to thisstructure, a region of the EL layer 222 with a small thickness can beeasily formed on the side surface of the spacer 215. This can preventundesired emission due to a current that flows through the EL layer 222between adjacent light-emitting elements. Providing the spacer 215having such a shape between light-emitting elements is effectiveparticularly when the pixel portion 11 has high definition because thedistance between adjacent light-emitting elements is reduced.Furthermore, that is effective particularly when the EL layer 222includes a layer containing highly conductive material, for example.

In the case where a blocking mask is used in the formation of the ELlayer 222, the second electrode 223, and the like, the spacer 215 mayhave a function of protecting the formation surface from flaws due tothe blocking mask.

The spacer 215 preferably overlaps with the wiring which intersects witha scan line.

A color filter method is used for the display device 10 shown in FIG. 10. For example, a structure in which one color is expressed by subpixelseach including the coloring layer 232 of any of red (R), green (G), andblue (B) may be used. In addition, subpixels for white (W) and yellow(Y) are preferably used because the color reproducibility can beimproved and power consumption can be reduced.

Owing to the combination of the coloring layer 232 and a microcavitystructure using the optical adjustment layer 224 in the light-emittingelement 254, light with high color purity can be extracted from thedisplay device 10. The thickness of the optical adjustment layer 224 isdetermined depending on the color of a subpixel. The optical adjustmentlayer may be omitted in some subpixels.

An EL layer that emits white light is preferably used as the EL layer222 of the light-emitting element 254. The use of the light-emittingelement 254 eliminates the need of separately coloring the EL layers 222of the subpixels, which leads to a reduction in cost and an increase inyield. In addition, the pixel portion 11 can be easily formed with highresolution. The subpixels may include optical adjustment layers havingdifferent thicknesses so that the EL layers 222 in the subpixels areseparately colored, in which case one or both of the optical adjustmentlayer and the coloring layer can be omitted. In that case, layers in thesubpixels are not necessarily colored separately except light-emittinglayers of the EL layers 222.

In the example shown in FIG. 10 , an FPC 242 is electrically connectedto the terminal portion 15. Thus, the display device 10 shown in FIG. 10can be referred to as a display module. A display device without an FPCor the like can be referred to as a display panel.

The terminal portion 15 is electrically connected to the FPC 242 throughthe connection layer 243.

The terminal portion 15 shown in FIG. 10 has a layered structureincluding the wiring 16 b and a conductive layer formed of the sameconductive film as the pixel electrode 225. The terminal portion 15preferably has a layered structure including a plurality of conductivelayers because not only a reduction in electric resistance but also anincrease in mechanical strength can be achieved.

It is preferred that the insulating layer 211 and the insulating layer221 be formed using a material through which impurities such as waterand hydrogen do not easily diffuse. That is, the insulating layer 211and the insulating layer 221 can serve as barrier films. With such astructure, entry of impurities from the outside into the light-emittingelement 254, the transistors, and the like can be effectively inhibitedeven when a moisture-permeable material is used for the first substrate201 and the second substrate 202, which leads to a highly reliabledisplay device.

The example shown in FIG. 10 has a sealed hollow structure including aspace 250 between the first substrate 201 and the second substrate 202.For example, the space 250 may be filled with an inert gas such asnitrogen or a rare gas. The space 250 may be filled with a fluidmaterial such as oil, or the pressure in the space 250 may be reduced.The sealing method is not limited thereto, and solid sealing using aresin or the like may be used.

Cross-Sectional Structure Example 2

FIG. 11 is a structure example of a display device which is suitablewhen the pixel portion 11 and the signal line driver circuit 13 are bentand used.

The display device 10 shown in FIG. 11 has a solid sealing structure inwhich the first substrate 201 and the second substrate 202 are bonded toeach other with a sealant 260.

A bonding layer 261 is provided over the first substrate 201. Aninsulating layer 216 is provided over the bonding layer 261. Atransistor, a light-emitting element, and the like are provided over theinsulating layer 216. The insulating layer 216 is preferably formedusing a material through which impurities such as water or hydrogen donot easily diffuse, like the insulating layer 221.

A bonding layer 262 is provided between the second substrate 202 and theinsulating layer 221.

As shown in FIG. 11 , the insulating layer 213 has an opening located ina portion closer to the outer end of the first substrate 201 than thepixel portion 11 and the signal line driver circuit 13. It is preferableto form an opening in the insulating layer 213 formed using a resinmaterial, for example, so as to surround the pixel portion 11, thesignal line driver circuit 13, and the like. In such a structure, thevicinity of the side surface of the insulating layer 213 which is incontact with the outside of the display device 10 does not form acontinuous layer with the region overlapping with the pixel portion 11,the signal line driver circuit 13, and the like, so that diffusion ofimpurities, such as water and hydrogen, from the outside through theinsulating layer 213 can be inhibited.

The solid sealing structure shown in FIG. 11 makes it easier to keep thedistance between the first substrate 201 and the second substrate 202constant. Thus, flexible substrates can be preferably used as the firstsubstrate 201 and the second substrate 202. As a result, part of or thewhole of the pixel portion 11, the scan line driver circuit 12, and thesignal line driver circuit 13 can be bent when used. For example, thedisplay device 10 can be bonded to a curved surface or the pixel portionof the display device 10 can be folded to produce electronic deviceswith a variety of structures.

Modification Example

An example of a touch panel including a touch sensor will be describedbelow.

FIG. 12 shows an example of a touch panel in which an on-cell touchsensor is used in the structure shown as an example in FIG. 10 .

Over the second substrate 202, a conductive layer 291 and a conductivelayer 292 are covered with an insulating layer 294. A conductive layer293 is provided over the insulating layer 294. The conductive layer 293is electrically connected, through an opening of the insulating layer294, to two conductive layers 292 between which the conductive layer 291is provided. The insulating layer 294 is bonded to a substrate 296 witha bonding layer 295.

The amount of the capacitance formed between the conductive layers 291and 292 changes with the approach of an object, so that the approach orcontact of the object can be sensed. A lattice arrangement of theplurality of conductive layers 291 and the plurality of conductivelayers 292 allows location information to be obtained.

A terminal portion 299 is provided in the vicinity of the outer end ofthe second substrate 202. The terminal portion 299 is electricallyconnected to an FPC 297 through a connection layer 298.

The substrate 296 here can be used also as a substrate with which anobject, such as a finger or a stylus, is to be in contact. In that case,a protective layer (such as a ceramic coat) is preferably provided overthe substrate 296. The protective layer can be formed using an inorganicinsulating material such as silicon oxide, aluminum oxide, yttriumoxide, or yttria-stabilized zirconia (YSZ). Alternatively, temperedglass may be used as the substrate 296. Physical or chemical processingby an ion exchange method, a wind tempering method, or the like may beperformed on tempered glass so that compressive stress is applied on thesurface. In the case where the touch sensor is provided on one side oftempered glass and the opposite side of the tempered glass is providedon, for example, the outermost surface of an electronic device for useas a touch surface, the thickness of the whole device can be decreased.

As the touch sensor, a capacitive touch sensor can be used. Examples ofthe capacitive touch sensor are a surface capacitive touch sensor and aprojected capacitive touch sensor. Examples of the projected capacitivetouch sensor include a self-capacitive touch sensor and a mutualcapacitive touch sensor. The use of a mutual capacitive type ispreferable because multiple points can be sensed simultaneously. Anexample of using a projected capacitive touch sensor will be describedbelow.

Note that one embodiment of the present invention is not limited to thisexample, and any of a variety of sensors capable of sensing the approachor contact of an object, such as a finger or a stylus, can be used.

The above example shows an on-cell touch panel in which a wiring and thelike composing a touch sensor are formed on the outer surface of thesecond substrate 202; there is no need to limit to the structure. Forexample, an external touch panel or an in-cell touch panel can beemployed. The use of the on-sell or in-cell touch panel allows areduction in the thickness of a display panel even when the displaypanel has a touch-panel function.

The above is the description of the cross-sectional structure examples.

Components

The above components will be described below.

Substrate

A substrate having a flat surface can be used as the substrate includedin the display device. The substrate through which light emitted fromthe light-emitting element is extracted is formed using a material thattransmits the light. For example, a material such as glass, quartz,ceramic, sapphire, or an organic resin can be used.

The weight and thickness of the display device can be reduced using athin substrate. Furthermore, a flexible display device can be obtainedusing a substrate that is thin enough to have flexibility.

As the glass, for example, alkali-free glass, barium borosilicate glass,aluminoborosilicate glass, or the like can be used.

Examples of a material having flexibility and transmitting visible lightinclude glass that is thin enough to have flexibility, polyester resinssuch as polyethylene terephthalate (PET) and polyethylene naphthalate(PEN), a polyacrylonitrile resin, a polyimide resin, a polymethylmethacrylate resin, a polycarbonate (PC) resin, a polyethersulfone (PES)resin, a polyamide resin, a cycloolefin resin, a polystyrene resin, apolyamide imide resin, a polyvinyl chloride resin, and apolytetrafluoroethylene (PTFE) resin. In particular, a material whosethermal expansion coefficient is low is preferred, and for example, apolyamide imide resin, a polyimide resin, or PET can be suitably used.Alternatively, a substrate in which a glass fiber is impregnated with anorganic resin or a substrate whose thermal expansion coefficient isreduced by mixing an organic resin with an inorganic filler can be used.A substrate using such a material is lightweight, and thus, a displaydevice using this substrate can also be lightweight.

Since the substrate through which light is not extracted does not needto have a light-transmitting property, a metal substrate or the like canbe used, other than the above-mentioned substrates. A metal substrate,which has high thermal conductivity, is preferable because it can easilyconduct heat to the whole substrate and accordingly can prevent a localtemperature rise in the display device.

Although there is no particular limitation on a material of the metalsubstrate, it is preferable to use, for example, a metal such asaluminum, copper, or nickel, or an alloy such as an aluminum alloy orstainless steel.

It is possible to use a substrate subjected to insulation treatment insuch a manner that a surface of a metal substrate is oxidized or aninsulating film is formed on a surface. An insulating film may be formedby, for example, a coating method such as a spin-coating method or adipping method, an electrodeposition method, an evaporation method, or asputtering method. An oxide film may be formed on the substrate surfaceby an anodic oxidation method, exposure to or heating in an oxygenatmosphere, or the like.

A hard coat layer (e.g., a silicon nitride layer) by which a surface ofthe display device is protected from damage, a layer (e.g., an aramidresin layer) that can disperse pressure, or the like may be stacked overthe flexible substrate. Furthermore, to suppress a decrease in thelifetime of the light-emitting element due to moisture and the like, aninsulating film with low water permeability may be stacked over theflexible substrate. For example, an inorganic insulating material suchas silicon nitride, silicon oxynitride, aluminum oxide, or aluminumnitride can be used.

The substrate may be formed by stacking a plurality of layers. Inparticular, when a glass layer is used, a barrier property against waterand oxygen can be improved, and thus, a highly reliable display devicecan be provided. For example, a substrate in which a glass layer, abonding layer, and an organic resin layer are stacked in this order fromthe side closer to the light-emitting element can be used. By providingsuch an organic resin layer, a crack or a break in the glass layer canbe suppressed and mechanical strength can be improved. The use of such acomposite material of a glass material and an organic resin for thesubstrate enables fabrication of a highly reliable and flexible displaydevice.

Transistor

The transistor included in the display device includes a conductivelayer functioning as a front gate electrode, a conductive layerfunctioning as a back gate electrode, the semiconductor layer, aconductive layer functioning as a source electrode, a conductive layerfunctioning as a drain electrode, and an insulating layer functioning asa gate insulating layer.

That is, in the transistor included in the display device of oneembodiment of the present invention, gate electrodes are provided overand under a channel.

There is no particular limitation on the crystallinity of asemiconductor material used for the transistor, and an amorphoussemiconductor or a semiconductor having crystallinity (amicrocrystalline semiconductor, a polycrystalline semiconductor, asingle crystal semiconductor, or a semiconductor partly includingcrystal regions) may be used. A semiconductor having crystallinity ispreferably used, in which case deterioration of the transistorcharacteristics can be suppressed.

As a semiconductor material used for the semiconductor layer of thetransistor, an oxide semiconductor can be used, for example. Inparticular, an oxide semiconductor having a wider band gap than siliconis preferably used. A semiconductor material having a wider band gap anda lower carrier density than silicon is preferably used because theoff-state current of the transistor can be reduced.

For example, the oxide semiconductor preferably contains at least indium(In) or zinc (Zn). The oxide semiconductor more preferably includes anIn-M-Zn-based oxide (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La,Ce, or Hf).

As the semiconductor layer, it is particularly preferable to use anoxide semiconductor layer including a plurality of crystal parts whosec-axes are aligned substantially perpendicularly to a surface on whichthe semiconductor layer is formed or the top surface of thesemiconductor layer and in which a grain boundary is not observedbetween adjacent crystal parts.

There is no grain boundary in such an oxide semiconductor; therefore,generation of a crack in an oxide semiconductor layer which is caused bystress when a display panel is bent is prevented. Consequently, such anoxide semiconductor can be preferably used for a flexible display devicewhich is used in a bent state, or the like.

Moreover, the use of such an oxide semiconductor with crystallinity forthe semiconductor layer makes it possible to provide a highly reliabletransistor in which a variation in electrical characteristics issuppressed.

A transistor with an oxide semiconductor whose band gap is larger thanthat of silicon can hold charge accumulated in a capacitor that isserially-connected to the transistor for a long time, owing to the lowoff-state current of the transistor. When such a transistor is used fora pixel, operation of a driver circuit can be stopped while the grayscale of each pixel is maintained. As a result, a display device withextremely low power consumption can be obtained.

Conductive Layer

As materials for the gates, the source, and the drain of a transistor,and the conductive layers serving as the wirings and electrodes includedin the display device, any of metals such as aluminum, titanium,chromium, nickel, copper, yttrium, zirconium, molybdenum, silver,tantalum, and tungsten, or an alloy containing any of these metals asits main component can be used. A single-layer structure or a layeredstructure including a film containing any of these materials can beused. For example, the following structures can be given: a single-layerstructure of an aluminum film containing silicon, a two-layer structurein which an aluminum film is stacked over a titanium film, a two-layerstructure in which an aluminum film is stacked over a tungsten film, atwo-layer structure in which a copper film is stacked over acopper-magnesium-aluminum alloy film, a two-layer structure in which acopper film is stacked over a titanium film, a two-layer structure inwhich a copper film is stacked over a tungsten film, a three-layerstructure in which a titanium film or a titanium nitride film, analuminum film or a copper film, and a titanium film or a titaniumnitride film are stacked in this order, and a three-layer structure inwhich a molybdenum film or a molybdenum nitride film, an aluminum filmor a copper film, and a molybdenum film or a molybdenum nitride film arestacked in this order. Note that an oxide such as indium oxide, tinoxide, or zinc oxide may be used. Copper containing manganese ispreferably used because controllability of a shape by etching isincreased.

As a light-transmitting material that can be used for the conductivelayers serving as the wirings and electrodes in the display device, aconductive oxide such as indium oxide, indium tin oxide, indium zincoxide, zinc oxide, or zinc oxide to which gallium is added, or graphenecan be used. Alternatively, a metal material such as gold, silver,platinum, magnesium, nickel, tungsten, chromium, molybdenum, iron,cobalt, copper, palladium, or titanium or an alloy material containingany of these metal materials can be used. Alternatively, a nitride ofthe metal material (e.g., titanium nitride) or the like may be used. Inthe case of using the metal material or the alloy material (or thenitride thereof), the thickness is set small enough to allow lighttransmission. Alternatively, a layered film of any of the abovematerials can be used as the conductive layer. For example, a layeredfilm of indium tin oxide and an alloy of silver and magnesium ispreferably used because the conductivity can be increased.

Insulating Layer

As an insulating material that can be used for the insulating layers,the overcoat, the spacer, and the like, a resin such as acrylic orepoxy, a resin having a siloxane bond, such as a silicone resin, or aninorganic insulating material such as silicon oxide, silicon oxynitride,silicon nitride oxide, silicon nitride, or aluminum oxide can be used.

The light-emitting element is preferably provided between a pair ofinsulating films with low water permeability, in which case entry ofimpurities such as water into the light-emitting element can beinhibited. Thus, a decrease in device reliability can be suppressed.

As an insulating film with low water permeability, a film containingnitrogen and silicon, such as a silicon nitride film or a siliconnitride oxide film, a film containing nitrogen and aluminum, such as analuminum nitride film, or the like can be used. Alternatively, a siliconoxide film, a silicon oxynitride film, an aluminum oxide film, or thelike may be used.

For example, the amount of water vapor transmission of the insulatingfilm with low water permeability is lower than or equal to 1×10⁻⁵[g/(m²·day)], preferably lower than or equal to 1×10⁻⁶ [g/(m²·day)],more preferably lower than or equal to 1×10⁻⁷ [g/(m²·day)], still morepreferably lower than or equal to 1×10⁻⁸ [g/(m²·day)].

Bonding Layer, Sealant

As the bonding layer and the sealant, a variety of curable adhesives,e.g., a photo-curable adhesive such as an ultraviolet curable adhesive,a reactive curable adhesive, a thermosetting curable adhesive, and ananaerobic adhesive can be used. Examples of these adhesives include anepoxy resin, an acrylic resin, a silicone resin, a phenol resin, apolyimide resin, an imide resin, a polyvinyl chloride (PVC) resin, apolyvinyl butyral (PVB) resin, and an ethylene vinyl acetate (EVA)resin. In particular, a material with low moisture permeability, such asan epoxy resin, is preferred. Alternatively, atwo-component-mixture-type resin may be used. Still alternatively, anadhesive sheet or the like may be used.

Furthermore, the resin may include a drying agent. For example, asubstance that adsorbs moisture by chemical adsorption, such as oxide ofan alkaline earth metal (e.g., calcium oxide or barium oxide), can beused. Alternatively, a substance that adsorbs moisture by physicaladsorption, such as zeolite or silica gel, may be used. The drying agentis preferably included because it can inhibit entry of impurities suchas moisture into a functional element, leading to an improvement in thereliability of the display panel.

In addition, a filler with a high refractive index or a light-scatteringmember may be mixed into the resin, in which case the efficiency oflight extraction from the light-emitting element can be improved. Forexample, titanium oxide, barium oxide, zeolite, or zirconium can beused.

Light-Emitting Element

As the light-emitting element, a self-luminous element can be used, andan element whose luminance is controlled by current or voltage isincluded in the category of the light-emitting element. For example, alight-emitting diode (LED), an organic EL element, or an inorganic ELelement can be used.

The light-emitting element may be a top emission, bottom emission, ordual emission light-emitting element. A conductive film that transmitsvisible light is used as the electrode through which light is extracted.A conductive film that reflects visible light is preferably used as theelectrode through which light is not extracted.

The EL layer includes at least a light-emitting layer. In addition tothe light-emitting layer, the EL layer may further include a layercontaining any of a substance with a high hole-injection property, asubstance with a high hole-transport property, a hole-blocking material,a substance with a high electron-transport property, a substance with ahigh electron-injection property, a substance with a bipolar property (asubstance with a high electron- and hole-transport property), and thelike.

For the EL layer, either a low-molecular compound or a high-molecularcompound can be used, and an inorganic compound may alternatively beused. The layers included in the EL layer can be formed by any of thefollowing methods: an evaporation method (including a vacuum evaporationmethod), a transfer method, a printing method, an inkjet method, acoating method, and the like.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between a cathode and an anode, holes are injected tothe EL layer from the anode side and electrons are injected to the ELlayer from the cathode side. The injected electrons and holes arerecombined in the EL layer and a light-emitting substance contained inthe EL layer emits light.

In the case where a light-emitting element emitting white light is usedas the light-emitting element, the EL layer preferably contains two ormore kinds of light-emitting substances. For example, light-emittingsubstances are selected such that two or more light-emitting substancesemit complementary colors to obtain white light emission. Specifically,it is preferable to use two or more light-emitting substances selectedfrom light-emitting substances emitting light of red (R), green (G),blue (B), yellow (Y), orange (O), and the like and light-emittingsubstances emitting light containing two or more of spectral componentsof R, G, and B. The light-emitting element preferably emits light with aspectrum having two or more peaks in the wavelength range of a visiblelight region (e.g., 350 nm to 750 nm). The emission spectrum of amaterial that emits light having a peak in a yellow wavelength rangepreferably includes spectral components also in green and red wavelengthranges.

A light-emitting layer containing a light-emitting material that emitslight of one color and a light-emitting layer containing alight-emitting material that emits light of another color are preferablystacked in the EL layer. For example, the plurality of light-emittinglayers in the EL layer may be stacked in contact with each other or maybe stacked with a region not including any light-emitting materialtherebetween. For example, between a fluorescent layer and aphosphorescent layer, a region containing the same material as that inthe fluorescent layer or phosphorescent layer (for example, a hostmaterial or an assist material) and no light-emitting material may beprovided. This facilitates the manufacture of the light-emitting elementand reduces the drive voltage.

The light-emitting element may be a single element including one ELlayer or a tandem element in which a plurality of EL layers are stackedwith a charge generation layer therebetween.

For the conductive film that transmits visible light, for example,indium oxide, indium tin oxide (ITO), indium zinc oxide, zinc oxide, orzinc oxide to which gallium is added can be used. Alternatively, a filmof a metal material such as gold, silver, platinum, magnesium, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, ortitanium, an alloy containing any of these metal materials, a nitride ofany of these metal materials (e.g., titanium nitride), or the like canbe used when formed thin enough to have a light-transmitting property.Alternatively, a layered film of any of the above materials can be usedas the conductive layer. For example, a layered film of ITO and an alloyof silver and magnesium is preferably used because the conductivity canbe increased. Still alternatively, graphene or the like may be used.

For the conductive film that reflects visible light, for example, ametal material such as aluminum, gold, platinum, silver, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or analloy containing any of these metal materials can be used. Lanthanum,neodymium, germanium, or the like may be added to the metal material orthe alloy. Alternatively, an alloy containing aluminum (an aluminumalloy), such as an alloy of aluminum and titanium, an alloy of aluminumand nickel, or an alloy of aluminum and neodymium, or an alloycontaining silver, such as an alloy of silver and copper, an alloy ofsilver, palladium, and copper, or an alloy of silver and magnesium canbe used for the conductive film. An alloy containing silver and copperis preferable because of its high heat resistance. Furthermore, when ametal film or a metal oxide film is stacked in contact with an aluminumalloy film, oxidation of the aluminum alloy film can be suppressed.Examples of a material for the metal film or the metal oxide filminclude titanium and titanium oxide. Alternatively, the above conductivefilm that transmits visible light and a film containing a metal materialmay be stacked. For example, a layered film of silver and ITO or alayered film of an alloy of silver and magnesium and ITO can be used.

The conductive layers may each be formed by an evaporation method or asputtering method. Alternatively, a discharging method such as an inkjetmethod, a printing method such as a screen printing method, or a platingmethod may be used.

Note that the aforementioned light-emitting layer and layers containinga substance with a high hole-injection property, a substance with a highhole-transport property, a substance with a high electron-transportproperty, a substance with a high electron-injection property, asubstance with a bipolar property, and the like may include an inorganiccompound such as a quantum dot or a high molecular compound (e.g., anoligomer, a dendrimer, and a polymer). For example, when used for thelight-emitting layer, the quantum dot can function as a light-emittingmaterial.

The quantum dot may be a colloidal quantum dot, an alloyed quantum dot,a core-shell quantum dot, a core quantum dot, or the like. A quantum dotcontaining elements belonging to Groups 12 and 16, elements belonging toGroups 13 and 15, or elements belonging to Groups 14 and 16 may be used.Alternatively, a quantum dot containing an element such as cadmium,selenium, zinc, sulfur, phosphorus, indium, tellurium, lead, gallium,arsenic, or aluminum may be used.

Coloring Layer

Examples of a material that can be used for the coloring layers includea metal material, a resin material, and a resin material containing apigment or dye.

Light-Blocking Layer

Examples of a material that can be used for the light-blocking layerinclude carbon black, a metal oxide, and a composite oxide containing asolid solution of a plurality of metal oxides. A stack of filmscontaining the material for the coloring layer can also be used for thelight-blocking layer. For example, a layered structure of a filmcontaining a material of a coloring layer which transmits light of acertain color and a film containing a material of a coloring layer whichtransmits light of another color can be employed. It is preferred thatthe coloring layer and the light-blocking layer be formed using the samematerial because the same manufacturing apparatus can be used and theprocess can be simplified.

Connection Layer

As a connection layer connecting an FPC or an IC and a terminal, ananisotropic conductive film (ACF), an anisotropic conductive paste(ACP), or the like can be used.

The above is the description of the components.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 3

In this embodiment, an example of a manufacturing method of a displaydevice including a flexible substrate will be described.

Here, layers including a light-emitting element, a circuit, a wiring, anelectrode, an insulating layer, optical members such as a coloring layerand a light-blocking layer, and the like are collectively referred to asan element layer. The element layer includes, for example, alight-emitting element, and may additionally include a wiringelectrically connected to the light-emitting element or an element suchas a transistor used in a pixel or a circuit.

In addition, here, a flexible member which supports the element layer ata stage at which the light-emitting element is completed (themanufacturing process is finished) is referred to as a substrate.Examples of the substrate include an extremely thin film with athickness greater than or equal to 10 nm and less than or equal to 300μm and the like.

As a method for forming an element layer over a flexible substrateprovided with an insulating surface, typically, there are two methodsshown below. One of them is to directly form an element layer over aflexible substrate. The other method is to form an element layer over asupport substrate that is different from a flexible substrate and thento separate the element layer from the support substrate to transfer theelement layer to the substrate. In addition to the above two methods,there is a method in which an element layer is formed over a substratewhich does not have flexibility and the substrate is thinned bypolishing or the like to have flexibility, though the details are notdescribed here.

In the case where a material of the substrate can withstand heatingtemperature in a process for forming the element layer, It is preferredthat the element layer be formed directly over the substrate, in whichcase a manufacturing process can be simplified. At this time, theelement layer is preferably formed in a state where the substrate isfixed to the support substrate, in which case transfer thereof in anapparatus and between apparatuses can be easy.

In the case of employing the method in which the element layer is formedover a support substrate and then transferred to a substrate, first, aseparation layer and an insulating layer are stacked over the supportsubstrate, and then the element layer is formed over the insulatinglayer. Next, the element layer is separated from the support substrateand then transferred to the substrate. At this time, a material isselected such that separation occurs at the interface between thesupport substrate and the separation layer, at the interface between theseparation layer and the insulating layer, or in the separation layer.In the method, It is preferred that a material having high heatresistance be used for the support substrate or the separation layer, inwhich case the upper limit of the temperature applied when the elementlayer is formed can be higher, and an element layer including a morereliable element can be formed.

For example, a stacked layer of a layer containing a high-melting-pointmetal material, such as tungsten, and a layer containing an oxide of themetal material is used as the separation layer. Furthermore, a stackedlayer of a plurality of layers, such as a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, and the like is preferably used as the insulating layer over theseparation layer. Note that in this specification, oxynitride containsmore oxygen than nitrogen, and nitride oxide contains more nitrogen thanoxygen.

The element layer and the support substrate can be separated by applyingmechanical force, by etching the separation layer, by making a liquidpermeate the separation interface, or the like. Alternatively,separation can be performed by heating or cooling two layers of theseparation interface to utilize a difference in thermal expansioncoefficient.

When separation is started, It is preferred that a separation trigger beformed first so that the separation proceeds from the trigger. Theseparation trigger can be formed, for example, by locally heating partof the insulating layer or the separation layer with laser light or thelike or by physically cutting or making a hole through part of theinsulating layer or the separation layer with a sharp tool.

The separation layer is not necessarily provided in the case where theseparation can be performed at the interface between the supportsubstrate and the insulating layer.

For example, glass is used for the support substrate and an organicresin such as polyimide is used for the insulating layer, in which caseseparation can be performed at the interface between the glass and theorganic resin. The remaining organic resin such as polyimide can be usedfor the substrate.

Alternatively, a heat generation layer may be provided between thesupport substrate and the insulating layer formed of an organic resin,and separation may be performed at the interface between the heatgeneration layer and the insulating layer by heating the heat generationlayer. As the heat generation layer, any of a variety of materials suchas a material that generates heat by current supply, a material thatgenerates heat by absorbing light, and a material that generates heat byapplication of a magnetic field can be used. For example, asemiconductor, a metal, or an insulator can be selected for the heatgeneration layer.

An example of a specific manufacturing method will be described below.The manufacturing method described below enables fabrication of aflexible input/output device of one embodiment of the present inventionby changing a layer formed as a layer to be separated.

First, a formation substrate 301 is provided with an island-shapedseparation layer 303. Then, the separation layer 303 is provided with alayer 305 to be separated (FIG. 13A). In addition, a formation substrate321 is provided with an island-shaped separation layer 323. Then, theseparation layer 323 is provided with a layer 325 to be separated (FIG.13B).

Although an example in which the separation layer is formed to have anisland shape is described here, one embodiment of the present inventionis not limited to this example. In this step, as the material for theseparation layer, a material that allows separation at the interfacebetween the formation substrate and the separation layer, the interfacebetween the separation layer and the layer to be separated, or in theseparation layer when the layer to be separated is separated from theformation substrate. Although an example in which separation occurs atthe interface between the separation layer and the layer to be separatedis described in this embodiment, one embodiment of the present inventionis not limited to such an example and depends on materials used for theseparation layer and the layer to be separated. Note that in the casewhere the layer to be separated has a layered structure, a layer incontact with the separation layer is particularly referred to as a firstlayer.

For example, when the separation layer has a layered structure of atungsten film and a tungsten oxide film and separation occurs at theinterface between the tungsten film and the tungsten oxide film (or thevicinity of the interface), part of the separation layer (here, part ofthe tungsten oxide film) may remain on the separated layer. Moreover,the separation layer remaining on the separated layer may be removedafter separation.

As the formation substrate, a substrate having at least heat resistanceenough to withstand process temperature in a manufacturing process isused. As the formation substrate, for example, a glass substrate, aquartz substrate, a sapphire substrate, a semiconductor substrate, aceramic substrate, a metal substrate, a resin substrate, or a plasticsubstrate can be used.

In the case where a glass substrate is used as the formation substrate,an insulating film such as a silicon oxide film, a silicon oxynitridefilm, a silicon nitride film, or a silicon nitride oxide film ispreferably formed as a base film between the formation substrate and theseparation layer, in which case contamination from the glass substratecan be prevented.

The separation layer can be formed using an element selected fromtungsten, molybdenum, titanium, tantalum, niobium, nickel, cobalt,zirconium, zinc, ruthenium, rhodium, palladium, osmium, iridium, andsilicon; an alloy material containing any of the elements; a compoundmaterial containing any of the elements; or the like. The crystalstructure of a layer containing silicon may be amorphous,microcrystalline, or polycrystalline. Alternatively, a metal oxide suchas aluminum oxide, gallium oxide, zinc oxide, titanium dioxide, indiumoxide, indium tin oxide, indium zinc oxide, or an In—Ga—Zn oxide may beused. The separation layer is preferably formed using a high-meltingpoint metal material such as tungsten, titanium, or molybdenum, in whichcase the degree of freedom of the process for forming the layers to beseparated can be increased.

The separation layer can be formed by, for example, a sputtering method,a plasma CVD method, a coating method (including a spin coating method,a droplet discharge method, a dispensing method, and the like), or aprinting method. The thickness of the separation layer is, for example,greater than or equal to 10 nm and less than or equal to 200 nm,preferably greater than or equal to 20 nm and less than or equal to 100nm.

In the case where the separation layer has a single-layer structure, atungsten layer, a molybdenum layer, or a layer containing a mixture oftungsten and molybdenum is preferably formed. Alternatively, a layercontaining an oxide or an oxynitride of tungsten, a layer containing anoxide or an oxynitride of molybdenum, or a layer containing an oxide oran oxynitride of a mixture of tungsten and molybdenum may be formed.Note that a mixture of tungsten and molybdenum is an alloy of tungstenand molybdenum, for example.

In the case where the separation layer is formed to have a layeredstructure including a layer containing tungsten and a layer containingan oxide of tungsten, the layer containing an oxide of tungsten may beformed as follows: the layer containing tungsten is formed first and aninsulating film formed of an oxide is formed thereover, so that thelayer containing an oxide of tungsten is formed at the interface betweenthe tungsten layer and the insulating film. Alternatively, the layercontaining an oxide of tungsten may be formed by performing thermaloxidation treatment, oxygen plasma treatment, nitrous oxide (N₂O) plasmatreatment, treatment with a highly oxidizing solution such as ozonewater, or the like on the surface of the layer containing tungsten.Plasma treatment or heat treatment may be performed in an atmosphere ofoxygen, nitrogen, or nitrous oxide alone, or a mixed gas of any of thesegasses and another gas. Surface condition of the separation layer ischanged by the plasma treatment or heat treatment, whereby adhesionbetween the separation layer and the insulating film formed later can becontrolled.

Note that the separation layer is not necessarily provided in the casewhere separation at the interface between the formation substrate andthe layer to be separated is possible. For example, a glass substrate isused as the formation substrate, and an organic resin such as polyimide,polyester, polyolefin, polyamide, polycarbonate, or acrylic is formed incontact with the glass substrate. Next, adhesion between the formationsubstrate and the organic resin is increased by laser light irradiationor heat treatment. Then, an insulating film, a transistor, and the likeare formed over the organic resin. After that, separation at theinterface between the formation substrate and the organic resin can beperformed by performing laser light irradiation with higher energydensity than that of the above laser light irradiation or performingheat treatment at a higher temperature than that for the above heattreatment. Moreover, the interface between the formation substrate andthe organic resin may be soaked in a liquid to perform separation.

Since the insulating film, the transistor, and the like are formed overthe organic resin having low heat resistance in the above method, it isimpossible to expose the substrate to high temperatures in themanufacturing process. Note that a transistor using an oxidesemiconductor is not necessarily processed at high temperature and thuscan be favorably formed over the organic resin.

The organic resin may be used for a substrate of the device.Alternatively, the organic resin may be removed and another substratemay be bonded to an exposed surface of the layer to be separated withthe use of an adhesive. In addition, the organic resin may be bonded toanother substrate (a supporting film) using an adhesive.

Alternatively, separation at the interface between a metal layer and theorganic resin may be performed in the following manner: a metal layer isprovided between the formation substrate and the organic resin andcurrent is made to flow in the metal layer so that the metal layer isheated.

The insulating layer (the first layer) in contact with the separationlayer preferably has a single-layer structure or a multilayer structureincluding any of a silicon nitride film, a silicon oxynitride film, asilicon oxide film, a silicon nitride oxide film, and the like. Notethat a material for the insulating layer is not limited thereto, and anoptimum material can be selected depending on a material used for theseparation layer.

The insulating layer can be formed by a sputtering method, a plasma CVDmethod, a coating method, a printing method, or the like. For example,the insulating layer is formed at higher than or equal to 250° C. andlower than or equal to 400° C. by a plasma CVD method, whereby theinsulating layer can be a dense film with high moisture resistance. Thethickness of the insulating layer ranges preferably from 10 nm to 3000nm, more preferably from 200 nm to 1500 nm.

Next, the formation substrate 301 and the formation substrate 321 areattached to each other with a bonding layer 307 such that surfaces onwhich the layers to be separated are formed face each other, and thebonding layer 307 is cured (see FIG. 13C).

Note that the formation substrate 301 and the formation substrate 321are preferably attached to each other in a reduced-pressure atmosphere.

Note that although FIG. 13C illustrates the case where the separationlayer 303 and the separation layer 323 are different in size, separationlayers having the same size as illustrated in FIG. 13D may be used.

The bonding layer 307 is provided to overlap with the separation layer303, the layer 305, the layer 325, and the separation layer 323. Then,the end portion of the bonding layer 307 is preferably positioned inwardfrom at least the end portion of either the separation layer 303 or theseparation layer 323 (the separation layer which is desirably separatedfirst). Accordingly, strong adhesion between the formation substrate 301and the formation substrate 321 can be suppressed; thus, a decrease inyield of a subsequent separating process can be suppressed.

As the bonding layer 307, various curable adhesives such as a reactivecurable adhesive, a thermosetting adhesive, an anaerobic adhesive, and aphotocurable adhesive such as an ultraviolet curable adhesive can beused. Examples of these adhesives include an epoxy resin, an acrylicresin, a silicone resin, a phenol resin, a polyimide resin, an imideresin, a PVC resin, a PVB resin, and an EVA resin. A material with lowmoisture permeability, such as an epoxy resin, is particularlypreferred. For the adhesive, a material having fluidity low enough todispose the material only in a desired region is preferably used. Forexample, an adhesive sheet, a bonding sheet, or a sheet-like orfilm-like adhesive can be used, and an optical clear adhesive (OCA) filmcan be preferably used.

The adhesive may have adhesion before attachment or exhibit adhesionafter attachment by heating or light irradiation.

Furthermore, the resin may include a drying agent. For example, it ispossible to use a substance that adsorbs moisture by chemicaladsorption, such as oxide of an alkaline earth metal (e.g., calciumoxide or barium oxide), or a substance that adsorbs moisture by physicaladsorption, such as zeolite or silica gel. The drying agent ispreferably included, in which case it can suppress deterioration of thefunctional element due to entry of moisture in the air and can improvethe reliability of the device.

Next, a separation trigger is formed by laser irradiation (FIGS. 14A and14B).

Either the formation substrate 301 or the formation substrate 321 can beseparated first. In the case where the separation layers differ in size,a substrate over which a larger separation layer is formed may beseparated first or a substrate over which a smaller separation layer isformed may be separated first. In the case where an element such as asemiconductor element or a light-emitting element is formed only overone of the substrates, the substrate on the side where the element isformed may be separated first or the other substrate may be separatedfirst. Here, the formation substrate 301 is separated first.

A region where the bonding layer 307 in a cured state, the layer 305,and the separation layer 303 overlap with one another is irradiated withlaser light (see the arrow P1 in FIG. 14A).

Part of the first layer is removed; thus, the separation trigger can beformed (see a region surrounded by a dashed line in FIG. 14B). At thistime, not only the first layer but also the separation layer 303, thebonding layer 307, or another layer included in the layer 305 may bepartly removed.

It is preferred that laser light irradiation be performed from the sideof the substrate provided with the separation layer that is desirablyseparated. In the case where a region where the separation layer 303 andthe separation layer 323 overlap with each other is irradiated withlaser light, the formation substrate 301 and the separation layer 303can be selectively separated from each other by cracking only the layer305 of the layers 305 and 325 (see a region surrounded by the dottedline in FIG. 14B). Here, an example in which layers of the layer 305 arepartly removed is shown.

Then, the layer 305 and the formation substrate 301 are separated fromeach other from the formed separation trigger (FIGS. 14C and 14D).Consequently, the layer 305 can be transferred from the formationsubstrate 301 to the formation substrate 321.

For example, the layer 305 and the formation substrate 301 may beseparated from each other from the separation trigger with mechanicalforce (e.g., a separation process with a human hand or a gripper, or aseparation process by rotation of a roller).

The formation substrate 301 and the layer 305 may be separated from eachother by making a liquid such as water permeate the interface betweenthe separation layer 303 and the layer 305. A portion between theseparation layer 303 and the layer 305 absorbs a liquid through acapillarity action, facilitating separation. Furthermore, an adverseeffect on the functional element included in the layer 305 due to staticelectricity caused at separation (e.g., a phenomenon in which asemiconductor element is damaged by static electricity) can besuppressed.

Next, the exposed layer 305 is attached to a substrate 331 with abonding layer 333, and the bonding layer 333 is cured (FIG. 15A).

Note that the layer 305 and the substrate 331 are preferably attached toeach other in a reduced-pressure atmosphere.

Subsequently, a separation trigger is formed by laser light irradiation(FIGS. 15B and 15C).

A region where the bonding layer 333 in a cured state, the layer 325,and the separation layer 323 overlap with each other is irradiated withlaser light (see the arrow P2 in FIG. 15B). Part of the first layer isremoved; thus, the separation trigger can be formed (see a regionsurrounded by a dashed line in FIG. 15C. Here, an example in whichlayers of the layer 325 are partly removed is shown). At this time, notonly the first layer but also the separation layer 323, the bondinglayer 333, or another layer included in the layer 325 may be partlyremoved.

Laser light is preferably delivered toward the formation substrate 321provided with the separation layer 323.

Then, the layer 325 and the formation substrate 321 are separated fromeach other from the separation trigger (see FIG. 15D). Accordingly, thelayer 305 and the layer 325 can be transferred to the substrate 331.

After that, another substrate may be bonded to the layer 325.

The exposed layer 325 is attached to a substrate 341 with a bondinglayer 343, and the bonding layer 343 is cured (FIG. 16A). FIG. 16A showsan example in which an opening has been already formed in the substrate341.

In this manner, the separated layers can be sandwiched between the pairof flexible substrates.

After that, unnecessary end portions of the substrate 331, the substrate341, and the like may be cut and removed as shown in FIG. 16B. Part ofthe end portions of the layers 305 and 325 may be cut at the same time.

By the above method, a flexible device can be fabricated. The separatedlayers having the structure described in the above embodiment can beused to fabricate a flexible display device.

In the above method for manufacturing the display device of oneembodiment of the present invention, a pair of formation substrates eachprovided with a separation layer and a layer to be separated areattached to each other, and then, a separation trigger is formed bylaser light irradiation to make separation of the layer to be separatedfrom the separation layer easier. As a result, the yield of theseparation process can be improved.

In addition, separation is performed after the formation substrates eachprovided with the separated layer are attached to each other in advance,and then a substrate that is included in a device desired to befabricated can be attached to the separated layer. As described above,formation substrates having low flexibility can be attached to eachother when the layers to be separated are attached to each other; thus,alignment accuracy at the time of attachment can be improved comparedwith the case where flexible substrates are attached to each other.

As shown in FIG. 17A, the end portion of a separation region 351 of thelayer 305 is preferably positioned inward from the end portion of theseparation layer 303. This can improve the yield of the separationprocess. When there is a plurality of regions 351, the separation layer303 may be provided for each region 351 as shown in FIG. 17B, or theplurality of regions 351 may be provided over one separation layer 303as shown in FIG. 17C.

The above is the description of a manufacturing method of a flexibledisplay device.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 4

In this embodiment, examples of electronic devices that can include adisplay device of one embodiment of the present invention will bedescribed.

Electronic devices and lighting devices can be manufactured using thedisplay device of one embodiment of the present invention. Electronicdevices and lighting devices with high display quality can bemanufactured using the display device of one embodiment of the presentinvention. Electronic devices and lighting devices with favorableviewing angle characteristics can be manufactured using the displaydevice of one embodiment of the present invention. Electronic devicesand lighting devices with low power consumption can be manufacturedusing the display device of one embodiment of the present invention. Inaddition, highly reliable electronic devices and highly reliablelighting devices can be manufactured using the display device of oneembodiment of the present invention.

Examples of electronic devices include a television set, desktop andlaptop personal computers, monitors of a computer and the like, adigital camera, a digital video camera, a digital photo frame, a mobilephone, a portable game machine, a portable information terminal, anaudio reproducing device, and a large game machine such as a pachinkomachine.

The electronic device or the lighting device of one embodiment of thepresent invention can be incorporated along a curved inside/outside wallsurface of a house or a building or a curved interior/exterior surfaceof a car.

The electronic device of one embodiment of the present invention mayinclude a secondary battery. Preferably, the secondary battery iscapable of being charged by contactless power transmission.

Examples of the secondary battery include a lithium ion secondarybattery such as a lithium polymer battery (lithium ion polymer battery)using a gel electrolyte, a nickel-hydride battery, a nickel-cadmiumbattery, an organic radical battery, a lead-acid battery, an airsecondary battery, a nickel-zinc battery, and a silver-zinc battery.

The electronic device of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, a video,information, or the like can be displayed on a display portion. When theelectronic device includes an antenna and a secondary battery, theantenna may be used for contactless power transmission.

The electronic device of one embodiment of the present invention mayinclude a sensor (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays).

The electronic device of one embodiment of the present invention canhave a variety of functions such as a function of displaying a varietyof information (e.g., a still image, a moving image, and a text image)on the display portion, a touch panel function, a function of displayinga calendar, date, time, and the like, a function of executing a varietyof kinds of software (programs), a wireless communication function, anda function of reading out a program or data stored in a recordingmedium.

Furthermore, the electronic device including a plurality of displayportions can have a function of displaying image information mainly onone display portion while displaying text information mainly on anotherdisplay portion, a function of displaying a three-dimensional image bydisplaying images where parallax is considered on a plurality of displayportions, or the like. Furthermore, the electronic device including animage receiving portion can have a function of photographing a stillimage or a moving image, a function of automatically or manuallycorrecting a photographed image, a function of storing a photographedimage in a recording medium (an external recording medium or a recordingmedium incorporated in the electronic device), a function of displayinga photographed image on a display portion, or the like. Note that thefunctions of the electronic devices of embodiments of the presentinvention are not limited thereto, and the electronic devices can have avariety of functions.

FIGS. 18A to 18E illustrate examples of electronic devices eachincluding a display portion 7000 with a curved surface. Since thedisplay surface of the display portion 7000 is curved, images can bedisplayed on the curved display surface. The display portion 7000 mayhave flexibility.

The display portion 7000 is formed using the display device or the likeof one embodiment of the present invention. One embodiment of thepresent invention makes it possible to provide a highly reliableelectronic device with low power consumption and a curved displayportion.

FIGS. 18A and 18B illustrate examples of mobile phones. A mobile phone7100 illustrated in FIG. 18A and a mobile phone 7110 illustrated in FIG.18B each include a housing 7101, the display portion 7000, operationbuttons 7103, an external connection port 7104, a speaker 7105, amicrophone 7106, and the like. The mobile phone 7110 illustrated in FIG.18B also includes a camera 7107.

Each mobile phone includes a touch sensor in the display portion 7000.Operations such as making a call and inputting text can be performed bytouch on the display portion 7000 with a finger, a stylus, or the like.

With the operation buttons 7103, the power can be turned on or off. Inaddition, types of images displayed on the display portion 7000 can bechanged; for example, switching from a mail creation screen to a mainmenu screen can be performed.

When a detection device such as a gyroscope or an acceleration sensor isprovided inside the mobile phone, the direction of display on the screenof the display portion 7000 can be automatically changed by determiningthe orientation of the mobile phone (whether the mobile phone is placedhorizontally or vertically). Furthermore, the direction of display onthe screen can be changed by touch on the display portion 7000,operation with the operation button 7103, sound input using themicrophone 7106, or the like.

FIGS. 18C and 18D illustrate examples of portable information terminals.A portable information terminal 7200 illustrated in FIG. 18C and aportable information terminal 7210 illustrated in FIG. 18D each includea housing 7201 and the display portion 7000. Each of the portableinformation terminals may also include an operation button, an externalconnection port, a speaker, a microphone, an antenna, a camera, abattery, or the like. The display portion 7000 is provided with a touchsensor. The operation of the portable information terminal can beperformed by touching the display portion 7000 with a finger, a stylus,or the like.

Each of the portable information terminals illustrated in thisembodiment functions as, for example, one or more of a telephone set, anotebook, and an information browsing system. Specifically, the portableinformation terminals can each be used as a smartphone. Each of theportable information terminals illustrated in this embodiment is capableof executing, for example, a variety of applications such as mobilephone calls, e-mailing, reading and editing text, music reproduction,Internet communication, and a computer game.

The portable information terminals 7200 and 7210 can display text, imageinformation, and the like on their plurality of surfaces. For example,as illustrated in FIGS. 18C and 18D, three operation buttons 7202 can bedisplayed on one surface, and information 7203 indicated by a rectanglecan be displayed on another surface. FIG. 18C illustrates an example inwhich information is displayed on the top of the portable informationterminal. FIG. 18D illustrates an example in which information isdisplayed on the side of the portable information terminal. Informationmay be displayed on three or more surfaces of the portable informationterminal.

Examples of the information include notification from a socialnetworking service (SNS), display indicating reception of an e-mail oran incoming call, the title of an e-mail or the like, the sender of ane-mail or the like, the date, the time, remaining battery level, and thereception sensitivity of an antenna. Alternatively, the operationbutton, an icon, or the like may be displayed instead of theinformation.

For example, a user of the portable information terminal 7200 can seethe display (here, the information 7203) on the portable informationterminal 7200 put in a breast pocket of his/her clothes.

Specifically, a caller's phone number, name, or the like of an incomingcall is displayed in a position that can be seen from above the portableinformation terminal 7200. Thus, the user can see the display withouttaking out the portable information terminal 7200 from the pocket anddecide whether to answer the call.

FIG. 18E illustrates an example of a television set. In a television set7300, the display portion 7000 is incorporated into a housing 7301.Here, the housing 7301 is supported by a stand 7303.

The television set 7300 illustrated in FIG. 18E can be operated with anoperation switch of the housing 7301 or a separate remote controller7311. The display portion 7000 may include a touch sensor, and can beoperated by touch on the display portion 7000 with a finger or the like.The remote controller 7311 may be provided with a display portion fordisplaying data output from the remote controller 7311. With operationkeys or a touch panel of the remote controller 7311, channels and volumecan be controlled and a video displayed on the display portion 7000 canbe controlled.

Note that the television set 7300 is provided with a receiver, a modem,and the like. A general television broadcast can be received with thereceiver. When the television set is connected to a communicationnetwork with or without wires via the modem, one-way (from a transmitterto a receiver) or two-way (between a transmitter and a receiver orbetween receivers) data communication can be performed.

FIG. 18F illustrates an example of a lighting device having a curvedlight-emitting portion.

The light-emitting portion included in the lighting device illustratedin FIG. 18F can be manufactured using the display device or the like ofone embodiment of the present invention. According to one embodiment ofthe present invention, a highly reliable lighting device with low powerconsumption and a curved light-emitting portion can be provided.

A light-emitting portion 7411 included in a lighting device 7400illustrated in FIG. 18F has two convex-curved light-emitting portionssymmetrically placed. Thus, light radiates from the lighting device 7400in all directions.

The light-emitting portion 7411 included in the lighting device 7400 mayhave flexibility. The light-emitting portion 7411 may be fixed on aplastic member, a movable frame, or the like so that a light-emittingsurface of the light-emitting portion 7411 can be bent freely dependingon the intended use.

The lighting device 7400 includes a stage 7401 provided with anoperation switch 7403 and the light-emitting portion 7411 supported bythe stage 7401.

Note that although the lighting device in which the light-emittingportion is supported by the stage is described as an example here, ahousing provided with a light-emitting portion can be fixed on a ceilingor suspended from a ceiling. Since the light-emitting surface can becurved, the light-emitting surface is curved to have a concave shape,whereby a particular region can be brightly illuminated, or thelight-emitting surface is curved to have a convex shape, whereby a wholeroom can be brightly illuminated.

FIGS. 19A to 19I illustrate examples of portable information terminalseach including a flexible and bendable display portion 7001.

The display portion 7001 is manufactured using the display device or thelike of one embodiment of the present invention. For example, a displaydevice or the like that can be bent with a radius of curvature ofgreater than or equal to 0.01 mm and less than or equal to 150 mm can beused. The display portion 7001 may include a touch sensor so that theportable information terminal can be operated by touch on the displayportion 7001 with a finger or the like. One embodiment of the presentinvention makes it possible to provide a highly reliable electronicdevice including a display portion having flexibility.

FIGS. 19A and 19B are perspective views illustrating an example of theportable information terminal. A portable information terminal 7500includes a housing 7501, the display portion 7001, a display portion tab7502, operation buttons 7503, and the like.

The portable information terminal 7500 includes the rolled flexibledisplay portion 7001 in the housing 7501. The display portion 7001 canbe pulled out with the display portion tab 7502.

The portable information terminal 7500 can receive a video signal with acontrol portion incorporated therein and can display the received videoon the display portion 7001. The portable information terminal 7500incorporates a battery. A terminal portion for connecting a connectormay be included in the housing 7501 so that a video signal and power canbe directly supplied from the outside with a wiring.

By pressing the operation buttons 7503, power ON/OFF, changing ofdisplayed videos, and the like can be performed. Although FIGS. 19A and19B show an example in which the operation buttons 7503 are positionedon a side surface of the portable information terminal 7500, oneembodiment of the present invention is not limited thereto. Theoperation buttons 7503 may be placed on a display surface (a frontsurface) or a rear surface of the portable information terminal 7500.

FIG. 19B illustrates the portable information terminal 7500 in a statewhere the display portion 7001 is pulled out. A video can be displayedon the display portion 7001 in this state. In addition, the portableinformation terminal 7500 may perform different types of display in thestate where part of the display portion 7001 is rolled as shown in FIG.19A and in the state where the display portion 7001 is pulled out asshown in FIG. 19B. For example, in the state shown in FIG. 19A, therolled portion of the display portion 7001 is put in a non-displaystate, reducing the power consumption of the portable informationterminal 7500.

Note that a reinforcement frame may be provided for a side portion ofthe display portion 7001 so that the display portion 7001 has a flatdisplay surface when pulled out.

Note that in addition to this structure, a speaker may be provided forthe housing so that sound is output with an audio signal receivedtogether with a video signal.

FIGS. 19C to 19E illustrate an example of a foldable portableinformation terminal. FIG. 19C illustrates a portable informationterminal 7600 that is unfolded. FIG. 19D illustrates the portableinformation terminal 7600 that is being unfolded or folded. FIG. 19Eillustrates the portable information terminal 7600 that is folded. Theportable information terminal 7600 is highly portable when folded, andis highly browsable when unfolded because of a seamless large displayarea.

The display portion 7001 is supported by three housings 7601 joinedtogether by hinges 7602. By folding the portable information terminal7600 at a connection portion between two housings 7601 with the hinges7602, the portable information terminal 7600 can be reversibly changedin shape from an unfolded state to a folded state.

FIGS. 19F and 19G illustrate an example of a foldable portableinformation terminal. FIG. 19F illustrates a portable informationterminal 7650 that is folded so that the display portion 7001 is on theinside. FIG. 19G illustrates the portable information terminal 7650 thatis folded so that the display portion 7001 is on the outside. Theportable information terminal 7650 includes the display portion 7001 anda non-display portion 7651. When the portable information terminal 7650is not used, the portable information terminal 7650 is folded so thatthe display portion 7001 is on the inside, whereby the display portion7001 can be prevented from being contaminated and damaged.

FIG. 19H illustrates an example of a flexible portable informationterminal. A portable information terminal 7700 includes a housing 7701and the display portion 7001. The portable information terminal 7700 mayfurther include buttons 7703 a and 7703 b serving as input portions,speakers 7704 a and 7704 b serving as sound output portions, an externalconnection port 7705, a microphone 7706, or the like. A flexible battery7709 can be included in the portable information terminal 7700. Thebattery 7709 may be arranged to overlap with the display portion 7001,for example.

The housing 7701, the display portion 7001, and the battery 7709 haveflexibility. Thus, it is easy to curve the portable information terminal7700 into a desired shape and to twist the portable information terminal7700. For example, the portable information terminal 7700 can be foldedso that the display portion 7001 is on the inside or on the outside. Theportable information terminal 7700 can be used in a rolled state. Sincethe housing 7701 and the display portion 7001 can be transformed freelyin this manner, the portable information terminal 7700 is less likely tobe broken even when the portable information terminal 7700 falls down orexternal stress is applied to the portable information terminal 7700.

The portable information terminal 7700 is lightweight and therefore canbe used conveniently in various situations. For example, the portableinformation terminal 7700 can be used in the state where the upperportion of the housing 7701 is suspended by a clip or the like, or inthe state where the housing 7701 is fixed to a wall by magnets or thelike.

FIG. 19I illustrates an example of a wrist-watch-type portableinformation terminal. The portable information terminal 7800 includes aband 7801, the display portion 7001, an input/output terminal 7802,operation buttons 7803, and the like. The band 7801 has a function of ahousing. A flexible battery 7805 can be included in the portableinformation terminal 7800. The battery 7805 may be provided to overlapwith the display portion 7001, the band 7801, or the like, for example.

The band 7801, the display portion 7001, and the battery 7805 haveflexibility. Thus, the portable information terminal 7800 can be easilycurved to have a desired shape.

With the operation buttons 7803, a variety of functions such as timesetting, ON/OFF of the power, ON/OFF of wireless communication, settingand cancellation of a silent mode, and setting and cancellation of apower saving mode can be performed. For example, the functions of theoperation buttons 7803 can be set freely by the operating systemincorporated in the portable information terminal 7800.

By touch on an icon 7804 displayed on the display portion 7001 with afinger or the like, an application can be started.

The portable information terminal 7800 can employ near fieldcommunication conformable to a communication standard. For example,mutual communication between the portable information terminal and aheadset capable of wireless communication can be performed, and thushands-free calling is possible.

The portable information terminal 7800 may include the input/outputterminal 7802. In the case where the input/output terminal 7802 isincluded in the portable information terminal 7800, data can be directlytransmitted to and received from another information terminal via aconnector. Charging through the input/output terminal 7802 is alsopossible. Note that charging of the portable information terminaldescribed as an example in this embodiment can be performed bycontactless power transmission without using the input/output terminal.

FIG. 20A is an external view of an automobile 7900. FIG. 20B illustratesa driver's seat of the automobile 7900. The automobile 7900 includes acar body 7901, wheels 7902, a windshield 7903, lights 7904, fog lamps7905, and the like.

The display device of one embodiment of the present invention can beused in a display portion of the automobile 7900. For example, thedisplay device of one embodiment of the present invention can be used indisplay portions 7910 to 7917 illustrated in FIG. 20B.

The display portion 7910 and the display portion 7911 are provided inthe automobile windshield. The display device of one embodiment of thepresent invention can be a see-through device, through which theopposite side can be seen, by using a light-transmitting conductivematerial for its electrodes. Such a see-through display device does nothinder driver's vision during the driving of the automobile 7900.Therefore, the display device of one embodiment of the present inventioncan be provided in the windshield of the automobile 7900. Note that inthe case where a transistor or the like is provided in the displaydevice, a transistor having light-transmitting properties, such as anorganic transistor using an organic semiconductor material or atransistor using an oxide semiconductor, is preferably used.

A display portion 7912 is provided on a pillar portion. A displayportion 7913 is provided on a dashboard. For example, the displayportion 7912 can compensate for the view hindered by the pillar portionby showing an image taken by an imaging portion provided on the carbody. Similarly, the display portion 7913 can compensate for the viewhindered by the dashboard and a display portion 7914 can compensate forthe view hindered by the door. That is, showing a video taken by animaging portion provided on the outside of the automobile leads toelimination of blind areas and enhancement of safety. In addition,showing a video so as to compensate for the area which a driver cannotsee makes it possible for the driver to confirm safety easily andcomfortably.

The display portion 7917 is provided in a steering wheel. The displayportion 7915, the display portion 7916, or the display portion 7917 candisplay a variety of kinds of information such as navigationinformation, a speedometer, a tachometer, a mileage, a fuel meter, agearshift indicator, and air-condition setting. The content, layout, orthe like of the display on the display portions can be customized freelyby a user as appropriate. The information listed above can also bedisplayed on the display portions 7910 to 7914.

The display portions 7910 to 7917 can also be used as lighting devices.

A display portion included in the display device of one embodiment ofthe present invention may have a flat surface. In that case, the displaydevice of one embodiment of the present invention does not necessarilyhave a curved surface and flexibility.

FIGS. 20C and 20D illustrate examples of digital signages. The digitalsignages each include a housing 8000, a display portion 8001, a speaker8003, and the like. Also, the digital signages can each include an LEDlamp, operation keys (including a power switch or an operation switch),a connection terminal, a variety of sensors, a microphone, and the like.

FIG. 20D illustrates a digital signage mounted on a cylindrical pillar.

A larger display portion 8001 can provide more information at a time. Inaddition, a larger display portion 8001 attracts more attention, so thatthe effectiveness of the advertisement is expected to be increased, forexample.

It is preferable to use a touch panel in the display portion 8001because a device with such a structure does not just display a still ormoving image, but can be operated by users intuitively. Alternatively,in the case where the device is used to provide information such asroute information or traffic information, usability can be enhanced byintuitive operation.

FIG. 20E illustrates a portable game console including a housing 8101, ahousing 8102, a display portion 8103, a display portion 8104, amicrophone 8105, a speaker 8106, an operation key 8107, a stylus 8108,and the like.

The portable game console illustrated in FIG. 20E includes two displayportions 8103 and 8104. Note that the number of display portions of theelectronic device of one embodiment of the present invention is notlimited to two and can be one or three or more as long as at least onedisplay portion includes the display device of one embodiment of thepresent invention.

FIG. 20F illustrates a laptop personal computer which includes a housing8111, a display portion 8112, a keyboard 8113, a pointing device 8114,and the like.

The display device of one embodiment of the present invention can beused for the display portion 8112.

FIG. 21A is an external view of a camera 8400 to which a finder 8500 isattached.

The camera 8400 includes a housing 8401, a display portion 8402, anoperation button 8403, a shutter button 8404, and the like. Furthermore,an attachable lens 8406 is attached to the camera 8400.

Although the lens 8406 of the camera 8400 here is detachable from thehousing 8401 for replacement, the lens 8406 may be built into a housing.

When the shutter button 8404 is pressed, the camera 8400 can takeimages. In addition, the display portion 8402 has a function of a touchpanel, and images can be taken when the display portion 8402 is touched.

The housing 8401 of the camera 8400 has a mount including an electrode,and the finder 8500, a stroboscope, and the like can be connected.

The finder 8500 includes a housing 8501, a display portion 8502, abutton 8503, and the like.

The housing 8501 includes a mount for engagement with the mount of thecamera 8400 so that the finder 8500 can be connected to the camera 8400.The mount includes an electrode, and a moving image or the like receivedfrom the camera 8400 through the electrode can be displayed on thedisplay portion 8502.

The button 8503 serves as a power button. The display portion 8502 canbe turned on and off using the button 8503.

A display device of one embodiment of the present invention can be usedfor the display portion 8402 of the camera 8400 and the display portion8502 of the finder 8500.

Although the camera 8400 and the finder 8500 are separate and detachableelectronic devices in FIG. 21A, a finder including the display device ofone embodiment of the present invention may be built into the housing8401 of the camera 8400.

FIG. 21B is an external view of a head-mounted display 8200.

The head-mounted display 8200 includes a mounting portion 8201, a lens8202, a main body 8203, a display portion 8204, a cable 8205, and thelike. In addition, a battery 8206 is built into the mounting portion8201.

Power is supplied from the battery 8206 to the main body 8203 throughthe cable 8205. The main body 8203 includes a wireless receiver or thelike to receive video data such as image data and display it on thedisplay portion 8204. The movement of the user's eyeball or eyelid iscaptured by a camera in the main body 8203 and then the coordinates ofthe eyepoint are calculated using the captured data to utilize theuser's eye as an input portion.

A plurality of electrodes may be provided in in a portion of themounting portion 8201 a user touches. The main body 8203 may have afunction of sensing a current flowing through the electrodes with themovement of the user's eyeball to determine the location of theeyepoint. The main body 8203 may have a function of sensing a currentflowing through the electrodes to monitor the user's pulse. The mountingportion 8201 may include sensors such as a temperature sensor, apressure sensor, or an acceleration sensor so that the user's biologicalinformation can be displayed on the display portion 8204. The main body8203 may sense the movement of the user's head or the like to move animage displayed on the display portion 8204 in synchronization with themovement of the user's head, or the like.

The display device of one embodiment of the present invention can beused for the display portion 8204.

FIGS. 21C and 21D are external views of a head-mounted display 8300.

The head-mounted display 8300 includes a housing 8301, two displayportions 8302, an operation button 8303, and a fixing band 8304.

The head-mounted display 8300 has the functions of the above-describedhead-mounted display 8200 and includes two display portions.

Since the head-mounted display 8300 includes the two display portions8302, the user's eyes can see their respective display portions. Thus, ahigh-definition image can be displayed even when a three-dimensionaldisplay using parallax, or the like, is performed. In addition, thedisplay portion 8302 is curved around an arc with the user's eye as anapproximate center. Owing to this, the distance between the user's eyeand the display surface of the display portion is uniform; thus, theuser can see a more natural image. Even when the luminance orchromaticity of light emitted from the display portion varies dependingon the user' viewing angle, the influence of the variation can besubstantially ignorable and thus a more realistic image can be displayedbecause the user's eye is positioned in the normal direction of thedisplay surface of the display portion.

The operation button 8303 serves as a power button or the like. A buttonother than the operation button 8303 may be included.

As illustrated in FIG. 21E, lenses 8305 may be provided between thedisplay portion 8302 and the user's eyes. The user can see magnifiedimages on the display portion 8302 through the lenses 8305, leading tohigher sense of presence. In that case, as illustrated in FIG. 21E, adial 8306 for changing the position of the lenses and adjustingvisibility may be included.

The display device of one embodiment of the present invention can beused for the display portion 8302. Since the display device of oneembodiment of the present invention has extremely high definition, evenwhen an image is magnified using the lenses 8305 as illustrated in FIG.21E, the pixels are not perceived by the user, and thus a more realisticimage can be displayed.

FIGS. 22A to 22C are examples in which the head-mounted display includesone display portion 8302. Such a structure can reduce the number ofcomponents.

The display portion 8302 can display an image for the right eye and animage for the left eye side by side on a right region and a left region,respectively. Thus, a three-dimensional moving image using binoculardisparity can be displayed.

One image which can be seen by both eyes may be displayed on all overthe display portion 8302. A panorama moving image can thus be displayedfrom end to end of the field of view; thus, the sense of reality isincreased.

As shown in FIG. 22C, the lenses 8305 may be provided. Two images may bedisplayed side by side on the display portion 8302. Alternatively, oneimage may be displayed on the display portion 8302 and seen by both eyesthrough the lenses 8305.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

EXAMPLE

The time required to charge and discharge wirings of a display device ofone embodiment of the present invention, and the like, were calculated.FIG. 26A is a block diagram of the display device used for thecalculation. FIG. 26B is a circuit diagram of a fabricated pixelcorresponding to the top view.

The block diagram of the display device in FIG. 26A illustrates aso-called 8K panel, which has a dimension of 65 inches and includes7680×4320 pixels (PIX) composed of subpixels for red, green, blue, andwhite (RGBW) arranged in stripes. Scan line driver circuits (GateDriver) provided on opposite sides are fabricated by gate on array (GOA)and output scan signals to the pixels (PIX). A signal line drivercircuit (Source Driver) is externally provided.

FIG. 26B is a circuit diagram of the pixel (PIX) illustrated in FIG.26A. The configuration of the circuit diagram in FIG. 26B corresponds tothe configuration in FIG. 7A in which the position of the capacitor C2is changed.

In the configuration in FIG. 26B, a first gate electrode and a secondgate electrode are not connected to each other in the transistor M4 asin FIG. 7A. With this configuration, the gate capacitance between a scanline GL and the transistor M4 is formed only between the scan line GLand the first gate electrode in contrast to a configuration where gateelectrodes are connected to each other.

FIG. 27 is a top view of the pixel (PIX) corresponding to FIG. 26B. Thepixel illustrated in FIG. 27 includes four subpixels for RGBW. Note thatcomponents corresponding to those in the circuit diagram are denoted bythe same reference symbols. The time required to charge and dischargewirings such as a scan line and a signal line was estimated using theconfigurations illustrated in FIGS. 26A and 26B and FIG. 27 . Variouscalculations to estimate the time required for charge and discharge wereconducted using the software SmartSpice produced by SILVACO, Inc. Notethat the pixel size is 188 μm×188 μm, the transistor M4 has a channellength L of 4 μm and a channel width W of 4 μm, and the transistor M5has a channel length L of 6 μm and a channel width W of 6 μm.

Table 1 shows the calculation results. In Table 1, “Gate fall time”refers to time required for a scan signal to fall, “Source line chargetime (>95%)” refers to time required to charge the signal line to 95%,“Total” refers to the total of Gate fall time and Source line chargetime, and “One horizontal period” refers to one horizontal scanningperiod.

TABLE 1 Gate fall Source line charge One horizontal time time(>95%)Total period 0.90 μs 0.97 μs 1.87 μs 1.92 μs

As shown in Table 1, time required to charge and discharge the scan lineand the signal line falls within one horizontal scanning period. Thisimplies that the display device of one embodiment of the presentinvention, in which the gate capacitance of the transistor connected tothe scan line is small, is suitable for an 8K panel.

EXPLANATION OF REFERENCE

GL: scan line, SL: signal line, V0: wiring, ANODE: current supply line,M1: transistor, M2: transistor, M3: transistor, C1: capacitor, EL:light-emitting element, CATHODE: common wiring, 100: transistor, 102:substrate, 104: insulating layer, 106: conductive layer, 108: oxidesemiconductor layer, 110: insulating layer, 112: oxide semiconductorlayer, 116: insulating layer, 108 i: channel region, 108 s: sourceregion, 108 d: drain region, 141 a: opening, 141 b: opening, 120 a:conductive layer, 120 b: conductive layer, 151: conductive layer, 152:conductive layer, 153: insulating layer, 161: oxide semiconductor layer,162: oxide semiconductor layer, 163: oxide semiconductor layer, 164:insulating layer, 171: oxide semiconductor layer, 172: oxidesemiconductor layer, 173: oxide semiconductor layer, 174: insulatinglayer, 181: conductive layer, 182: conductive layer, 183: conductivelayer, 184: conductive layer, 185: conductive layer, 186: insulatinglayer, 187: insulating layer, 190: opening, 191: conductive layer, 192:conductive layer, 193: insulating layer, 198: light-emitting layer, 199:partition layer, 10: display device, 11: pixel portion, 12: scan linedriver circuit, 13: signal line driver circuit, 15: terminal portion, 16a: wiring, 16 b: wiring, 22: region, 24: region, M4: transistor, M5:transistor, M6: transistor, M7: transistor, M8: transistor, M9:transistor, M10: transistor, M11: transistor, C2: capacitor, C3:capacitor, C4: capacitor, C5: capacitor, GL1: scan line, GL2: scan line,GL3: scan line, GL4: scan line, V1: wiring, V2: wiring, 201: substrate,202: substrate, 211: insulating layer, 212: insulating layer, 213:insulating layer, 214: insulating layer, 215: spacer, 216: insulatinglayer, 217: insulating layer, 218: insulating layer, 220: bonding layer,221: insulating layer, 222: EL layer, 223: electrode, 224: opticaladjustment layer, 225: pixel electrode, 230 a: structure, 230 b:structure, 231: light-blocking layer, 232: coloring layer, 242: FOP,243: connection layer, 250: space, 251: transistor, 252: transistor,253: capacitor, 254: light-emitting element, 255: transistor, 260:sealing material, 261: bonding layer, 262: bonding layer, 271:semiconductor layer, 272: conductive layer, 273: conductive layer, 274:conductive layer, 275: conductive layer, 276: insulating layer, 291:conductive layer, 292: conductive layer, 293: conductive layer, 294:insulating layer, 295: bonding layer, 296: substrate, 297: FOP, 298:connection layer, 299: terminal portion, 301: formation substrate, 303:separation layer, 305: layer to be separated, 307: bonding layer, 321:formation substrate, 323: separation layer, 325: layer to be separated,331: substrate, 333: bonding layer, 341: substrate, 343: bonding layer,351: region, 7000: display portion, 7001: display portion, 7100: mobilephone, 7101: housing, 7103: operation button, 7104: external connectionport, 7105: speaker, 7106: microphone, 7107: camera, 7110: mobile phone,7200: portable information terminal, 7201: housing, 7202: operationbutton, 7203: information, 7210: portable information terminal, 7300:television set, 7301: housing, 7303: stand, 7311: remote controller,7400: lighting device, 7401: stage, 7403: operation switch, 7411:light-emitting portion, 7500: portable information terminal, 7501:housing, 7502: display portion tab, 7503: operation button, 7600:portable information terminal, 7601: housing, 7602: hinge, 7650:portable information terminal, 7651: non-display portion, 7700: portableinformation terminal, 7701: housing, 7703 a: button, 7703 b: button,7704 a: speaker, 7704 b: speaker, 7705: external connection port, 7706:microphone, 7709: battery, 7800: portable information terminal, 7801:band, 7802: input-output terminal, 7803: operation button, 7804: icon,7805: battery, 7900: automobile, 7901: car body, 7902: wheel, 7903:windshield, 7904: light, 7905: fog lamp, 7910: display portion, 7911:display portion, 7912: display portion, 7913: display portion, 7914:display portion, 7915: display portion, 7916: display portion, 7917:display portion, 8000: housing, 8001: display portion, 8003: speaker,8101: housing, 8102: housing, 8103: display portion, 8104: displayportion, 8105: microphone, 8106: speaker, 8107: operation key, 8108:stylus, 8111: housing, 8112: display portion, 8113: keyboard, 8114:pointing device, 8200: head-mounted display, 8201: mounting portion,8202: lens, 8203: main body, 8204: display portion, 8205: cable, 8206:battery, 8300: head-mounted display, 8301: housing, 8302: displayportion, 8303: operation button, 8304: fixing band, 8305: lens, 8306:dial, 8400: camera, 8401: housing, 8402: display portion, 8403:operation button, 8404: shutter button, 8406: lens, 8500: finder, 8501:housing, 8502: display portion, 8503: button.

This application is based on Japanese Patent Application serial no.2015-256583 filed with Japan Patent Office on Dec. 28, 2015 and JapanesePatent Application serial no. 2016-218998 filed with Japan Patent Officeon Nov. 9, 2016, the entire contents of which are hereby incorporated byreference.

The invention claimed is:
 1. A display device comprising a pixel, thepixel comprising: a light-emitting element; a first transistor, one of asource and a drain of the first transistor being electrically connectedto a pixel electrode of the light-emitting element; a second transistor,one of a source and a drain of the second transistor being electricallyconnected to a signal line; and a third transistor, one of a source anda drain of the third transistor being electrically connected to awiring, wherein the other of the source and the drain of the firsttransistor is electrically connected to a current supply line, whereinthe other of the source and the drain of the second transistor iselectrically connected to a gate electrode of the first transistor,wherein the other of the source and the drain of the third transistor iselectrically connected to the pixel electrode, wherein the displaydevice further comprises: a first conductive layer functioning as thecurrent supply line; a second conductive layer functioning as the signalline; a third conductive layer functioning as the wiring, a fourthconductive layer electrically connected to the one of the source and thedrain of the second transistor; a fifth conductive layer electricallyconnected to the other of the source and the drain of the secondtransistor; a sixth conductive layer electrically connected to the otherof the source and the drain of the third transistor; and a seventhconductive layer, wherein the first conductive layer, the secondconductive layer, and the seventh conductive layer are positioned in thesame layer, wherein the third conductive layer, the fourth conductivelayer, the fifth conductive layer, and the sixth conductive layer arepositioned in the same layer, wherein the first conductive layer and thethird conductive layer overlap each other, wherein the first conductivelayer and the gate electrode of the first transistor overlap each other,wherein the first conductive layer and a gate electrode of the thirdtransistor overlap each other, and wherein the sixth conductive layer iselectrically connected to the pixel electrode through the seventhconductive layer.
 2. The display device according to claim 1, wherein agate electrode of the second transistor is electrically connected to afirst scan line, and wherein the gate electrode of the third transistoris electrically connected to a second scan line.
 3. The display deviceaccording to claim 1, wherein the first conductive layer and the secondconductive layer are positioned over the third conductive layer.
 4. Thedisplay device according to claim 1, wherein, when seen from above, thethird conductive layer has a region extending in the same direction asthe first conductive layer and the second conductive layer.
 5. Thedisplay device according to claim 1, wherein, when seen from above, anarea of the sixth conductive layer is larger than an area of the fourthconductive layer.
 6. The display device according to claim 1, wherein,when seen from above, an area of the sixth conductive layer is largerthan an area of the fifth conductive layer.
 7. A display devicecomprising a pixel, the pixel comprising: a light-emitting element; afirst transistor, one of a source and a drain of the first transistorbeing electrically connected to a pixel electrode of the light-emittingelement; a second transistor, one of a source and a drain of the secondtransistor being electrically connected to a signal line; and a thirdtransistor, one of a source and a drain of the third transistor beingelectrically connected to a wiring, wherein the other of the source andthe drain of the first transistor is electrically connected to a currentsupply line, wherein the other of the source and the drain of the secondtransistor is electrically connected to a gate electrode of the firsttransistor, wherein the other of the source and the drain of the thirdtransistor is electrically connected to the pixel electrode, wherein thedisplay device further comprises: a first conductive layer functioningas the current supply line; a second conductive layer functioning as thesignal line; a third conductive layer functioning as the wiring, afourth conductive layer electrically connected to the one of the sourceand the drain of the second transistor; a fifth conductive layerelectrically connected to the other of the source and the drain of thesecond transistor; and a sixth conductive layer electrically connectedto the other of the source and the drain of the third transistor,wherein the first conductive layer and the second conductive layer arepositioned in the same layer, wherein the third conductive layer, thefourth conductive layer, the fifth conductive layer, and the sixthconductive layer are positioned in the same layer, wherein the firstconductive layer and the third conductive layer overlap each other,wherein the first conductive layer and the gate electrode of the firsttransistor overlap each other, and wherein the first conductive layerand a gate electrode of the third transistor overlap each other.
 8. Thedisplay device according to claim 7, wherein a gate electrode of thesecond transistor is electrically connected to a first scan line, andwherein the gate electrode of the third transistor is electricallyconnected to a second scan line.
 9. The display device according toclaim 7, wherein the first conductive layer and the second conductivelayer are positioned over the third conductive layer.
 10. The displaydevice according to claim 7, wherein, when seen from above, the thirdconductive layer has a region extending in the same direction as thefirst conductive layer and the second conductive layer.
 11. The displaydevice according to claim 7, wherein, when seen from above, an area ofthe sixth conductive layer is larger than an area of the fourthconductive layer.
 12. The display device according to claim 7, wherein,when seen from above, an area of the sixth conductive layer is largerthan an area of the fifth conductive layer.